Aldactone: Brand name for spironolactone, a prescription potassium sparing diuretic with antiandrogen effects. It is commonly used off label in selected women with female pattern hair loss and other androgen related conditions.

Alopecia: The medical term for hair loss. Alopecia includes many different conditions and may be temporary or permanent, localized or diffuse, and scarring or non scarring.

Alopecia Areata: An autoimmune disease in which the immune system attacks hair follicles, causing non scarring hair loss. It commonly appears as smooth, round or oval patches but can progress to more extensive hair loss.

Alopecia Barbae: A form of alopecia areata that causes patchy hair loss in the beard area.

Alopecia Ophiasis: A pattern of alopecia areata characterized by band like hair loss around the sides and back of the scalp.

Alopecia Totalis: A severe form of alopecia areata resulting in complete or nearly complete loss of scalp hair.

Alopecia Universalis: A rare and extensive form of alopecia areata resulting in complete or nearly complete loss of hair across the scalp and body, including eyebrows and eyelashes.

Amino Acids: The building blocks of proteins, including keratin, which is a major structural component of hair. Severe protein or nutritional deficiencies can contribute to hair shedding, but amino acid supplementation has not been established as a treatment for common forms of hair loss in people without a deficiency.

Anagen: The active growth phase of the hair cycle, during which the hair follicle produces a growing hair shaft. Scalp hair can remain in anagen for several years.

Anagen Effluvium: Rapid hair loss caused by interruption of actively growing anagen hairs. It is most commonly associated with chemotherapy, radiation and certain toxic exposures and is usually non scarring.

Androgen: A class of steroid hormones that includes testosterone and dihydrotestosterone, or DHT. Androgens play an important role in sexual development and are also involved in androgenetic alopecia.

Androgen Receptor: A cellular receptor that binds androgens such as testosterone and DHT. Androgen receptor signaling in genetically susceptible scalp hair follicles plays a central role in the follicular miniaturization seen in androgenetic alopecia.

Androgenetic Alopecia: The most common form of progressive hair loss in men and women, also known as male pattern hair loss or female pattern hair loss. It occurs in genetically susceptible individuals and involves progressive miniaturization of affected hair follicles in response to androgen signaling.

Anterior: Toward the front. In hair restoration, the term is commonly used when describing the frontal or anterior scalp.

Antiandrogen: A medication or substance that reduces androgen production or blocks the effects of androgens on target tissues. Certain antiandrogens are used under medical supervision in the treatment of androgen related hair loss.

Aromatase: An enzyme that converts androgens into estrogens. Aromatase is present in human hair follicles and may influence the local balance between androgen and estrogen activity within scalp tissue.

Autograft: Tissue transplanted from one location to another in the same person. Hair follicles moved during hair transplant surgery are autografts.

Azelaic Acid: A naturally occurring dicarboxylic acid commonly used to treat acne and rosacea. Although laboratory research has suggested potential effects on 5 alpha reductase activity, azelaic acid has not been established or approved as a treatment for androgenetic alopecia.

Baldness: A general, nonmedical term used to describe visible or progressive hair loss. It is most commonly associated with androgenetic alopecia, but baldness itself is not a specific medical diagnosis.

Baricitinib: An oral Janus kinase, or JAK, inhibitor marketed as Olumiant. It is FDA approved for the treatment of adults with severe alopecia areata and works by reducing immune signaling involved in the disease.

Beard Transplantation: A hair restoration procedure in which donor hair follicles, usually taken from the scalp, are transplanted to areas of the beard or mustache to increase density, restore missing facial hair or reconstruct areas affected by scarring.

Benign Prostatic Hyperplasia (BPH): A noncancerous enlargement of the prostate gland that commonly occurs with aging. Finasteride and dutasteride, medications also used in the management of androgenetic alopecia, were originally developed and approved at specific doses for the treatment of BPH.

Bicalutamide: A prescription androgen receptor blocker primarily used in the treatment of prostate cancer. It is also used off label by some physicians in selected women with female pattern hair loss. Bicalutamide is not FDA approved for hair loss and requires appropriate medical supervision.

Biopsy: The removal of a small sample of tissue for microscopic examination. A scalp biopsy may be used when the cause of hair loss is unclear and can be particularly important in diagnosing scarring alopecias and other inflammatory scalp disorders.

Biotin: Also known as vitamin B7, biotin is an essential nutrient involved in normal metabolism. Biotin deficiency can cause hair loss, but deficiency is uncommon, and high quality evidence does not support routine biotin supplementation as a treatment for hair loss in people who are not deficient.

Body Hair Transplantation (BHT): A specialized form of hair transplantation in which follicles are harvested from areas such as the beard, chest or other parts of the body for transplantation to the scalp. Body hair differs from scalp hair in growth cycle, caliber and other characteristics and is generally considered an additional donor source rather than a substitute for adequate scalp donor hair.

Bonding: A method of attaching a nonsurgical hair replacement system or hairpiece to the scalp using adhesive, tape or another bonding material.

Bulb (Hair Bulb): The enlarged lower portion of the hair follicle surrounding the dermal papilla. It contains rapidly dividing matrix cells that produce the developing hair shaft.

Bulge Region: A specialized region of the hair follicle that contains an important population of hair follicle stem cells involved in follicular regeneration and normal hair cycling. Damage to this permanent portion of the follicle is particularly important in scarring forms of alopecia.

Catagen: The short transitional phase of the hair growth cycle that follows anagen, the active growth phase, and precedes telogen, the resting phase. During catagen, hair growth stops and the follicle begins to regress.

Central Centrifugal Cicatricial Alopecia (CCCA): A form of scarring hair loss that typically begins near the center or crown of the scalp and may gradually spread outward. It is diagnosed most commonly in Black women, although it can occur in men and people of other racial backgrounds. Early diagnosis and treatment are important because destruction of affected hair follicles can result in permanent hair loss.

Chemotherapy: The use of medications to destroy or slow the growth of cancer cells. Because some chemotherapy drugs also affect rapidly dividing cells within the hair follicle, treatment can cause temporary or, less commonly, persistent hair loss.

Chemotherapy Induced Alopecia (CIA): Hair loss caused by certain chemotherapy medications. The degree and pattern of hair loss depend on the drug, dose and treatment regimen. Hair commonly begins to regrow after chemotherapy ends, although changes in texture, color or density may occur.

Cicatricial Alopecia: Also known as scarring alopecia, a group of hair loss disorders in which inflammation damages and destroys hair follicles and replaces them with scar tissue. Because follicular destruction can be permanent, early diagnosis and treatment are particularly important.

Clascoterone: A topical androgen receptor inhibitor being investigated for the treatment of androgenetic alopecia. A 5 percent topical solution has completed Phase 3 clinical trials in men with pattern hair loss, but as of 2026 it remains investigational and is not FDA approved for hair loss.

Club Hair: A fully keratinized hair with a characteristic club shaped root that has completed active growth. Club hairs are normally shed during the hair cycle and are commonly seen during increased shedding associated with telogen effluvium.

Cobblestoning: An uneven or raised appearance of the recipient scalp that can result when hair transplant grafts are placed too high or fail to heal flush with the surrounding skin. It is most strongly associated with older plug and large graft hair transplant techniques.

Corticosteroids: Anti inflammatory medications that suppress immune activity. Topical, injected or systemic corticosteroids may be used in the treatment of certain autoimmune and inflammatory hair loss disorders, including alopecia areata and some forms of scarring alopecia.

Cortex: The thick middle layer of the hair shaft and its primary structural component. The cortex contains keratin fibers and pigment and contributes most of the hair shaft’s strength, elasticity and color.

Crown: The upper rear portion of the scalp, also commonly called the vertex. The crown is a frequent area of progressive thinning and hair loss in androgenetic alopecia.

Cuticle: The outermost protective layer of the hair shaft, composed of overlapping flattened cells. The cuticle helps protect the inner structure of the hair and influences its appearance, texture and resistance to damage.

Dermal Papilla: A specialized group of cells located at the base of the hair follicle that plays a central role in regulating hair follicle development, growth and cycling. Dermal papilla cells communicate with surrounding follicular cells through complex signaling pathways and are an important focus of hair regeneration research.

Dermis: The deeper layer of skin beneath the epidermis. The dermis contains connective tissue, blood vessels, nerves, sebaceous glands and much of the hair follicle structure.

Dermoscopy: A noninvasive diagnostic technique that uses magnification and specialized lighting to examine structures in the skin and scalp that are not easily visible to the naked eye. When used to evaluate hair and scalp disorders, dermoscopy is commonly referred to as trichoscopy.

Dihydrotestosterone (DHT): A potent androgen formed when testosterone is converted by the enzyme 5 alpha reductase. In genetically susceptible scalp hair follicles, DHT and androgen receptor signaling play a central role in the progressive follicular miniaturization associated with androgenetic alopecia.

Diffuse Patterned Alopecia (DPA): A form of androgenetic alopecia characterized by diffuse thinning across the areas of the scalp typically affected by pattern hair loss while the permanent donor region remains relatively preserved. It should be distinguished from diffuse unpatterned alopecia because the distinction can significantly affect hair transplant candidacy.

Diffuse Unpatterned Alopecia (DUPA): An uncommon pattern of diffuse hair loss in which follicular miniaturization extends throughout the scalp, including areas normally considered part of the permanent donor region. Because the donor area may also be affected, people with DUPA are generally poor candidates for hair transplant surgery unless donor stability can be clearly established.

Direct Hair Implantation (DHI): A term commonly used to describe hair transplant graft placement with an implanter device that can create a recipient site and place a follicular unit in a single step. The donor follicles are typically harvested using FUE. DHI is an implantation technique and should not be interpreted as a fundamentally separate method of donor harvesting or a completely different form of hair transplantation.

Donor Area: The region of the scalp from which hair follicles are harvested for transplantation. In properly selected patients, the most reliable donor hair generally comes from areas that are relatively resistant to the progressive miniaturization associated with androgenetic alopecia.

Donor Depletion: The reduction of available donor hair caused by previous hair transplant harvesting. Excessive or poorly planned extraction can create visible thinning, scarring and an unnatural donor appearance while permanently limiting the number of follicles available for future procedures.

Donor Density: The number of follicular units or hairs within a given area of donor scalp, commonly measured per square centimeter. Donor density is an important factor in determining how much hair may be safely harvested and the amount of coverage that can realistically be achieved through hair transplantation.

Donor Dominance: The principle that transplanted hair follicles tend to retain the genetic characteristics of the area from which they were taken after being moved to another part of the scalp. This principle forms the biological basis of hair transplantation, but long term stability depends on harvesting follicles from a genuinely stable donor region.

Donor Safe Zone: The portion of the scalp traditionally considered most resistant to androgenetic hair loss and therefore the preferred source of permanent donor hair for transplantation. The size and boundaries of this zone vary between individuals, and follicles harvested outside a stable donor area may miniaturize over time.

Double Blind Study: A clinical study in which neither the participants nor the researchers directly evaluating them know who is receiving the treatment being studied and who is receiving a placebo or comparison treatment. This design helps reduce bias when assessing treatment safety and effectiveness.

Drug Induced Hair Loss: Hair loss triggered by a medication or other therapeutic agent. Depending on the drug and mechanism involved, shedding may occur through telogen effluvium, anagen effluvium or other pathways. Hair growth often improves after the responsible medication is changed or discontinued, although this should only be done under medical supervision.

Dutasteride: A prescription 5 alpha reductase inhibitor that blocks both type 1 and type 2 forms of the enzyme responsible for converting testosterone to DHT. Dutasteride is FDA approved in the United States for benign prostatic hyperplasia and is used off label by some physicians to treat androgenetic alopecia. It is a potent and long acting medication that should be used under medical supervision.

Ectodermal Dysplasia: A group of inherited disorders affecting structures that develop from the ectoderm, including the hair, teeth, nails and sweat glands. Certain forms can cause sparse, thin or absent scalp and body hair.

Effluvium: A general medical term describing abnormal or excessive hair shedding. Examples include telogen effluvium and anagen effluvium. The term describes the shedding process rather than a single specific diagnosis.

Epidermis: The outermost layer of the skin. Hair shafts emerge through the epidermis, while most of the hair follicle extends into deeper layers of the skin.

Estradiol: The most biologically active form of estrogen during the reproductive years. Estradiol influences many tissues in the body, including hair follicles, although its relationship to hair growth and hair loss is complex.

Estrogen: A class of steroid hormones involved in reproductive and numerous other biological functions. Changes in estrogen levels can influence the hair growth cycle and may contribute to changes in hair density during pregnancy, after childbirth and around menopause.

Exclamation Point Hairs: Short, broken hairs that are narrower near the scalp and wider toward the tip, creating an appearance similar to an exclamation point. They are a characteristic clinical finding often associated with active alopecia areata.

Exogen: The shedding phase of the hair cycle during which a club hair is released from the follicle. Exogen is now recognized as a distinct process rather than simply part of the telogen resting phase.

Exosomes: Very small extracellular vesicles released by cells that carry proteins, lipids and genetic material involved in cell to cell communication. Exosome based products are being promoted and investigated for hair loss and regenerative medicine, but there are currently no FDA approved exosome products in the United States, including for the treatment of hair loss.

Eyebrow Transplantation: A specialized hair restoration procedure in which donor hair follicles, usually taken from the scalp, are transplanted into the eyebrow area to restore or increase eyebrow density. Careful planning of hair direction, angle and growth characteristics is particularly important because transplanted scalp hair retains many of its original characteristics.

Female Pattern Hair Loss (FPHL): The most common form of progressive hair loss in women. It typically causes gradual thinning across the central and upper scalp and is influenced by genetic, hormonal and age related factors. The term female pattern hair loss is generally preferred over the older term female pattern baldness.

Fibrosing Alopecia in Pattern Distribution (FAPD): A form of scarring alopecia that combines features of patterned hair loss with inflammatory follicular destruction. It can resemble androgenetic alopecia but may show redness, scaling and progressive scarring around affected follicles.

5 Alpha Reductase: An enzyme that converts testosterone into dihydrotestosterone, or DHT. DHT plays a central role in androgenetic alopecia in genetically susceptible hair follicles.

5 Alpha Reductase Inhibitors: A class of medications that reduce the conversion of testosterone to DHT by inhibiting the enzyme 5 alpha reductase. Finasteride and dutasteride are the best known examples.

Finasteride: A prescription type 2 5 alpha reductase inhibitor that reduces the conversion of testosterone to DHT. Finasteride 1 mg is FDA approved for male pattern hair loss, while the 5 mg dose is approved for benign prostatic hyperplasia. Finasteride is also used off label in selected patients under medical supervision.

Flap Surgery: A historical hair restoration procedure in which a section of hair bearing scalp was partially detached and repositioned into a bald area while remaining connected to its original blood supply. Flap procedures are now rarely performed because of the risk of unnatural appearance, scarring and other complications.

Follicle: A complex miniature organ within the skin from which a hair grows. The hair follicle includes several specialized structures involved in hair production, cycling and regeneration.

Follicular Unit: A naturally occurring grouping of usually one to four hairs that grow together from the scalp along with associated sebaceous glands, nerves and supporting structures.

Follicular Unit Excision (FUE), formerly commonly called Follicular Unit Extraction: A hair transplant donor harvesting technique in which individual follicular units are surgically excised with small punches and removed one at a time from the donor area. FUE does not create a linear donor scar, but it does create numerous small scars and is not a scarless procedure. The formal terminology was changed from extraction to excision to more accurately describe the surgical nature of the technique.

FUE Overharvesting: Excessive removal of follicular units from the donor area during FUE surgery. Overharvesting can produce visible thinning, an uneven or moth eaten donor appearance, scarring and permanent loss of donor hair that may be needed for future procedures.

Follicular Unit Graft: A hair transplant graft consisting of a single naturally occurring follicular unit. Depending on the individual unit, it may contain one or several hairs.

Follicular Unit Transplantation (FUT): A hair transplant technique in which a strip of donor scalp is surgically removed and dissected under magnification into individual follicular unit grafts for transplantation. FUT generally leaves a linear donor scar and is also commonly referred to as strip surgery.

Folliculitis: Inflammation of one or more hair follicles. It can result from infection, irritation, trauma or inflammatory skin disease and may appear as redness, tenderness, itching or small bumps around affected follicles.

Frontal Fibrosing Alopecia (FFA): A form of scarring alopecia that typically causes progressive recession of the frontal and temporal hairline and may also involve eyebrow loss. Because affected follicles can be permanently destroyed, early recognition and treatment are important.

Free Flap: A historical surgical hair restoration technique in which a section of hair bearing scalp was completely detached and transferred to another area of the scalp using microsurgical reconnection of blood vessels. The procedure is rarely used today.

Gene Therapy: A treatment strategy that modifies, replaces, adds or regulates genetic material in an effort to treat disease. Gene based approaches are being studied in hair follicle biology and hair loss research, but gene therapy is not currently an established or FDA approved treatment for common forms of hair loss.

Genetic Predisposition: An inherited tendency to develop a particular condition. Androgenetic alopecia has a strong genetic component involving multiple genes, and susceptibility can be inherited from either side of the family.

Graft: Hair bearing tissue transplanted from a donor area to a recipient area during hair restoration surgery. In modern hair transplantation, a graft most commonly consists of a naturally occurring follicular unit containing one or more hairs.

Graft Dissection: The process of preparing donor tissue into individual follicular unit grafts for transplantation. In FUT surgery, donor strip tissue is typically dissected under magnification to minimize follicular damage and preserve intact follicular units.

Graft Placement: The insertion of prepared follicular unit grafts into recipient sites created in areas of hair loss. Proper control of angle, direction, depth and distribution is critical to achieving natural appearing hair growth and protecting graft viability.

Graft Survival: The successful healing and continued growth of transplanted hair follicles after surgery. Graft survival can be influenced by surgical technique, handling, hydration, trauma, time outside the body and conditions within the recipient area.

Graft Transection: Accidental cutting or damage to part of a hair follicle during donor harvesting or graft preparation. Excessive transection can reduce the number of viable follicles available for transplantation and may negatively affect surgical yield.

Growth Factors: Naturally occurring signaling proteins that influence cell growth, repair, inflammation and tissue regeneration. Growth factors are frequently discussed in connection with treatments such as platelet rich plasma and regenerative therapies, but the presence of growth factors alone does not establish that a treatment is clinically effective for hair loss.

Gynecomastia: Enlargement of male breast glandular tissue, usually related to changes in the balance or action of hormones. Gynecomastia can occur naturally and may also be associated with certain medications or medical conditions.

Hair Caliber: The diameter or thickness of an individual hair shaft. Hair caliber varies between individuals and across different areas of the scalp and may progressively decrease when follicles miniaturize in androgenetic alopecia.

Hair Cloning: A general term used to describe experimental approaches intended to create additional hair follicles or follicle producing cells for transplantation or regeneration. Despite longstanding research, true hair cloning is not currently an established clinical treatment for hair loss.

Hair Cycle: The repeating biological process through which hair follicles move through phases of active growth, transition, rest and shedding. The primary phases are anagen, catagen, telogen and exogen.

Hair Density: The number of hairs or follicular units within a given area of scalp, commonly measured per square centimeter. Hair density is an important consideration in diagnosing hair loss and planning hair restoration surgery.

Hair Follicle: The complex miniature organ within the skin that produces hair. It contains specialized structures and cell populations that regulate hair formation, growth, cycling and regeneration.

Hair Integration: A nonsurgical hair replacement method in which additional human or synthetic hair is incorporated into a person’s existing hair. The term is sometimes used interchangeably with hair weaving or similar attachment systems.

Hairline Design: The planning of the shape, height, density and direction of a reconstructed frontal hairline during hair transplant surgery. Appropriate hairline design should account for age, facial proportions, future hair loss and available donor hair.

Hairlift: A historical scalp reduction procedure in which large areas of bald scalp were surgically removed and hair bearing scalp was advanced to reduce the size of the bald area. The procedure is rarely performed today because of significant scarring, distortion and other potential complications.

Hair Matrix: A region of rapidly dividing cells within the lower portion of the hair follicle that produces the hair shaft and contributes to formation of the inner root sheath.

Hair Miniaturization: The progressive reduction in the size and diameter of affected hair follicles and hair shafts. Miniaturization is a defining feature of androgenetic alopecia and can eventually result in hairs becoming increasingly fine and less visible.

Hair Multiplication: An experimental concept involving the expansion or replication of hair follicle cells or structures to create additional hair growth or transplantable follicles. Although research continues, hair multiplication is not currently an established clinical treatment.

Hair Pull Test: A simple clinical examination in which a small group of hairs is gently pulled to assess excessive shedding. The test may help evaluate active hair loss but must be interpreted together with the patient’s history and other clinical findings.

Hair Shaft: The visible portion of hair that extends above the surface of the skin. It is composed primarily of keratin and consists of the cuticle, cortex and, in some hairs, a central medulla.

Hair Transplantation: A surgical procedure in which permanent hair follicles are moved from a donor area to areas affected by hair loss. Modern hair transplantation most commonly uses follicular unit grafts harvested by FUT or FUE.

Hair Weaving: A nonsurgical hair replacement technique in which human or synthetic hair is attached to existing hair through braiding, weaving or other attachment methods. The added hair does not grow from the scalp.

Hamilton Norwood Scale: A classification system used to describe the progression and patterns of male androgenetic alopecia. It ranges from minimal frontal recession to extensive hair loss involving the frontal, mid scalp and crown regions.

Hirsutism: Excessive growth of coarse terminal hair in women in areas where hair growth is typically associated with androgen activity, such as the face, chest or abdomen. Hirsutism may be associated with elevated androgen levels or increased sensitivity to androgens.

Hormone: A chemical messenger produced by glands or other tissues that travels through the body and influences specific cells and organs. Hormones including androgens, estrogens and thyroid hormones can influence hair growth and the hair cycle.

Hyperandrogenism: A state of excessive androgen production or activity. In women, hyperandrogenism may be associated with conditions such as polycystic ovary syndrome and can contribute to hirsutism, acne and female pattern hair loss.

Hypertrichosis: Excessive hair growth that is not limited to androgen dependent areas and can occur in men, women or children. Hypertrichosis may be genetic, associated with medical conditions or caused by certain medications, including minoxidil.

Hypothyroidism: A condition in which the thyroid gland does not produce enough thyroid hormone. Hypothyroidism can contribute to diffuse hair thinning or increased shedding and may also affect the texture and quality of the hair.

Hypotrichosis: A condition characterized by less hair growth than expected for a person’s age, sex or body area. Hypotrichosis may be congenital or acquired and can result from genetic disorders, medical conditions or damage to hair follicles.

Implanter Device: A surgical instrument used to place follicular unit grafts into recipient areas during hair transplantation. Implanter devices can allow the surgeon or surgical team to control graft depth, angle and direction during placement and are commonly associated with techniques marketed as DHI.

Immune Mediated Hair Loss: Hair loss caused by abnormal immune activity directed against hair follicles or structures surrounding them. Alopecia areata and several forms of scarring alopecia involve immune mediated mechanisms, although the specific biological processes differ between disorders.

Immunotherapy: A treatment approach that alters or redirects immune activity. In hair loss medicine, topical contact immunotherapy has been used for extensive alopecia areata by deliberately producing a controlled allergic reaction on the scalp in an effort to modify the immune response affecting the hair follicles.

Inflammation: The body’s biological response to injury, infection or abnormal immune activity. Inflammatory processes can affect the scalp and hair follicles and play an important role in several forms of alopecia, particularly scarring hair loss disorders.

Infundibulum: The upper portion of the hair follicle extending from the skin surface to the opening of the sebaceous gland duct. It is one of the major anatomical regions of the hair follicle.

Inner Root Sheath: A structure surrounding and supporting the developing hair shaft within the lower portion of the hair follicle. It helps guide and shape the growing hair before breaking down higher within the follicle.

Intermediate Hair: A hair whose diameter and other characteristics fall between those of a fine vellus hair and a thicker terminal hair. Intermediate hairs may be seen during progressive follicular miniaturization in androgenetic alopecia.

Intralesional Corticosteroids: Corticosteroid medication injected directly into affected skin or scalp. Intralesional corticosteroid injections are commonly used in the treatment of localized alopecia areata and may also be used in selected inflammatory scalp disorders under medical supervision.

Isthmus: The middle portion of the hair follicle located between the opening of the sebaceous gland duct and the attachment of the arrector pili muscle. This region includes important follicular structures associated with regeneration and stem cell activity.

Janus Kinase (JAK): A family of intracellular enzymes involved in signaling pathways that regulate immune and inflammatory activity. Abnormal JAK signaling plays an important role in several immune mediated diseases, including alopecia areata.

JAK Inhibitors: A class of medications that block one or more Janus kinase enzymes and reduce specific immune signaling pathways. Several JAK inhibitors are now used in the treatment of moderate to severe alopecia areata, representing one of the most important advances in medical therapy for the disease.

JAK STAT Pathway: A cellular signaling system involving Janus kinase, or JAK, enzymes and STAT proteins that transmits signals from cell surface receptors to the nucleus. This pathway plays an important role in immune regulation and has become a major therapeutic target in alopecia areata.

Juri Flap: A historical hair restoration procedure in which a long strip of hair bearing scalp was surgically rotated into a bald frontal area while remaining attached to its original blood supply. The technique is now rarely used because of extensive scarring, unnatural appearance and the availability of modern follicular unit transplantation techniques.

Keloid: A type of raised scar caused by excessive collagen formation during healing. Keloids can occur after surgery or skin injury and may be a consideration in patients with a personal or family tendency toward abnormal scarring.

Kenogen: A period in the hair cycle during which a follicle remains temporarily empty after a telogen hair has been shed and before a new anagen hair emerges. A prolonged kenogen phase may contribute to reduced visible hair density.

Keratin: A strong structural protein that forms the major component of the hair shaft, nails and outer layers of the skin.

Keratinization: The biological process through which cells produce keratin, mature and form hardened structures such as the hair shaft.

Keratinocyte: A cell that produces keratin. Specialized keratinocytes within the hair follicle play a major role in forming the growing hair shaft and surrounding follicular structures.

Ketoconazole: An antifungal medication available in topical formulations, including shampoos commonly used to treat dandruff and seborrheic dermatitis. Ketoconazole shampoo is sometimes used as an adjunct in people with androgenetic alopecia, particularly when scalp inflammation or seborrheic dermatitis is present, but it is not an FDA approved treatment for hair loss.

Lanugo Hair: Fine, soft, usually unpigmented hair that develops on the fetus during pregnancy and is normally shed before or shortly after birth. Lanugo like hair can occasionally appear later in life in association with certain medical conditions.

Latanoprost: A prostaglandin analog prescription medication primarily used to lower pressure inside the eye. Prostaglandin analogs can stimulate eyelash growth, and latanoprost has been studied experimentally for scalp hair growth, but it is not an established or FDA approved treatment for androgenetic alopecia.

Lichen Planopilaris (LPP): An inflammatory form of scarring alopecia in which immune activity damages and can permanently destroy hair follicles. Symptoms may include scalp redness, scaling, itching, burning or tenderness. Early diagnosis and treatment are important because hair lost after follicular destruction may not regrow.

Low Dose Oral Minoxidil (LDOM): The use of oral minoxidil at relatively low doses to treat hair loss. Oral minoxidil was originally developed as a blood pressure medication and is not FDA approved for hair loss, but it is increasingly prescribed off label by physicians for selected men and women with androgenetic alopecia and other hair loss disorders.

Low Level Light Therapy (LLLT): A form of photobiomodulation that uses low intensity red or near infrared light delivered through devices such as caps, helmets or combs. Some devices are FDA cleared for androgenetic alopecia, although device design, wavelength, treatment parameters and supporting clinical evidence vary considerably.

Loose Anagen Hair Syndrome: A hair growth disorder in which anagen hairs are poorly anchored within the follicle and can be easily and painlessly pulled from the scalp. It occurs most often in young children, particularly girls, and can cause short, thin or difficult to grow hair.

Ludwig Scale: A classification system used to describe the severity and progression of female pattern hair loss, particularly diffuse thinning over the central and upper scalp. The classic Ludwig system divides hair loss into three stages of increasing severity.

Lupus Related Alopecia: Hair loss associated with lupus erythematosus. Lupus may cause non scarring shedding or inflammatory scarring alopecia, including discoid lupus erythematosus of the scalp. Scarring forms can permanently destroy affected hair follicles.

Male Pattern Hair Loss (MPHL): The most common form of progressive hair loss in men and a form of androgenetic alopecia. It typically involves recession of the frontal hairline, thinning in the mid scalp and crown, and progressive follicular miniaturization in genetically susceptible areas.

Matrix Cells: Rapidly dividing cells located within the hair bulb that produce the growing hair shaft and contribute to formation of the inner root sheath.

Medulla: The innermost layer of some hair shafts. It is not present in every hair and generally has less structural importance than the cortex and cuticle.

Mesotherapy: A technique involving multiple small injections of medications, vitamins, peptides or other substances into the scalp or skin. Mesotherapy is marketed for hair loss in some settings, but formulations and protocols vary widely and it is not an established standard treatment for androgenetic alopecia.

Microneedling: A procedure that uses fine needles to create controlled microinjuries in the skin. Microneedling has been studied alone and in combination with treatments such as minoxidil for androgenetic alopecia, but protocols, needle depth and supporting evidence vary, and repeated injury may carry risks including irritation, infection and scarring.

Mid Scalp: The central portion of the scalp located between the frontal hairline and the crown or vertex. The mid scalp is a common area of thinning in androgenetic alopecia.

Miniaturization: The progressive reduction in the size of hair follicles and the diameter and length of the hairs they produce. Follicular miniaturization is a defining feature of androgenetic alopecia.

Minoxidil: A medication that promotes hair growth and prolongs the anagen phase of the hair cycle. Topical minoxidil is FDA approved for certain forms of pattern hair loss in men and women. Oral minoxidil is approved as a blood pressure medication and is used off label at lower doses for hair loss under medical supervision.

Monilethrix: A rare inherited hair shaft disorder in which hairs develop a characteristic beaded appearance and are prone to breakage. The condition can cause short, fragile hair and varying degrees of hair loss.

Mosaicism: The presence of two or more genetically distinct cell populations within the same individual. Certain forms of mosaicism can affect skin and hair and may contribute to localized or patterned hair abnormalities.

Nape Hair: Hair located along the lower back of the scalp and upper neck. Nape hair may be finer and less resistant to future androgenetic hair loss than follicles from the central permanent donor region, making careful assessment important before it is used for hair transplantation.

Neogenesis: The formation of new tissue or structures. In hair loss research, hair follicle neogenesis refers to efforts to generate entirely new hair follicles rather than simply preserve, enlarge or transplant existing follicles. True follicle neogenesis remains an area of experimental research.

Non Scarring Alopecia: A broad category of hair loss in which the hair follicle is not permanently destroyed by scar tissue. Examples include androgenetic alopecia, alopecia areata and telogen effluvium. Because the follicular structure remains present, regrowth may be possible depending on the underlying condition.

Norwood Scale: Common name for the Hamilton Norwood Scale, a classification system used to describe the progression and patterns of male androgenetic alopecia, ranging from minimal frontal recession to extensive loss involving the frontal scalp, mid scalp and crown.

Nutrient Deficiency Related Hair Loss: Hair shedding or impaired hair growth associated with inadequate levels of certain nutrients, including iron, protein, zinc or other essential nutrients. Nutritional deficiencies should be identified through appropriate medical evaluation rather than assumed based on hair loss alone.

Nutraceutical: A food derived product or supplement marketed for potential health benefits. Many nutraceuticals are promoted for hair growth, but ingredients, formulations and clinical evidence vary widely, and supplements should not automatically be considered effective treatments for hair loss.

Papilla: A small projection of tissue. In hair biology, the term commonly refers to the dermal papilla, a specialized structure at the base of the hair follicle that plays a central role in regulating hair growth and cycling.

Pattern Hair Loss: A general term for androgenetic alopecia in men and women. It is characterized by progressive follicular miniaturization occurring in recognizable patterns on the scalp and is influenced by genetics, androgen signaling and age.

Perifollicular Erythema: Redness surrounding individual hair follicles. It can be a sign of active inflammation and is an important clinical finding in several inflammatory and scarring forms of alopecia.

Perifollicular Scale: Scaling or flaking concentrated around the openings of individual hair follicles. Perifollicular scale may indicate follicular inflammation and is commonly evaluated when diagnosing conditions such as lichen planopilaris and frontal fibrosing alopecia.

Photobiomodulation: The use of low intensity light to influence cellular activity. In hair loss treatment, photobiomodulation generally refers to red or near infrared light delivered by devices marketed or cleared for androgenetic alopecia and is also commonly referred to as low level light therapy.

Placebo: An inactive substance or treatment designed to resemble the therapy being studied. Placebos are used in controlled clinical trials to help determine whether observed treatment effects are greater than changes that might occur without the active therapy.

Platelet Rich Plasma (PRP): A preparation made from a patient’s own blood in which platelets are concentrated and injected into the scalp. PRP contains growth factors and signaling proteins and is used by some physicians for androgenetic alopecia. Preparation methods and treatment protocols vary considerably, and clinical results are not uniform.

Polycystic Ovary Syndrome (PCOS): A common hormonal disorder that can involve irregular ovulation, elevated androgen activity, acne, hirsutism and female pattern hair loss. PCOS is one potential cause of androgen related hair changes in women and requires appropriate medical evaluation.

Poliosis: Localized whitening or loss of pigment in hair. Poliosis can occur with several autoimmune and inflammatory conditions and may occasionally be seen in association with alopecia areata.

Postpartum Hair Loss: Increased hair shedding that commonly occurs several months after childbirth as a larger than usual number of follicles enter the telogen phase following pregnancy. It is generally a form of telogen effluvium and usually improves over time.

Primary Cicatricial Alopecia: A group of inflammatory hair loss disorders in which the hair follicle itself is the primary target of destructive inflammation. Permanent scarring can result when follicles are replaced by fibrous tissue. Examples include lichen planopilaris, frontal fibrosing alopecia and central centrifugal cicatricial alopecia.

Propecia: Brand name for finasteride 1 mg, an FDA approved prescription medication for the treatment of male pattern hair loss. Finasteride reduces DHT by inhibiting type 2 5 alpha reductase.

Proscar: Brand name for finasteride 5 mg, FDA approved for the treatment of benign prostatic hyperplasia. Although the active medication is the same as in Propecia, the approved dose and indication are different.

Prostaglandin Analogs: A class of medications that affect prostaglandin signaling. Certain prostaglandin analogs can stimulate eyelash growth and have been investigated for scalp hair growth, although they are not established standard treatments for androgenetic alopecia.

Punch: A small cylindrical surgical instrument used to cut around tissue. In modern FUE hair transplantation, small punches are used to excise individual follicular units from the donor area.

Punch Graft: A historical hair transplant graft created by removing relatively large circular pieces of hair bearing scalp with a punch and transplanting them into bald areas. Large punch grafts often produced the unnatural plug like appearance associated with older hair transplant surgery and have been replaced by modern follicular unit techniques.

Radiation Induced Alopecia: Hair loss caused by radiation therapy or other significant radiation exposure involving the scalp. Depending on the radiation dose and extent of follicular injury, hair loss may be temporary or permanent.

Receding Hairline: Progressive movement of the frontal hairline backward, commonly beginning at the temples. Hairline recession is a frequent early feature of male androgenetic alopecia, although the degree and pattern vary between individuals.

Recipient Area: The area of the scalp into which hair follicle grafts are transplanted during hair restoration surgery. Recipient areas commonly include the frontal hairline, mid scalp and crown.

Recipient Site: A small incision or opening created in the recipient area for placement of a follicular unit graft. The angle, direction, distribution and density of recipient sites are important factors in achieving a natural appearing hair transplant result.

Regenerative Medicine: A broad field focused on repairing, replacing or regenerating damaged cells and tissues. In hair loss, the term is frequently used in connection with treatments such as platelet rich plasma, cellular therapies, growth factors and exosomes. Many treatments marketed as regenerative medicine remain experimental or have varying levels of clinical evidence.

Ritlecitinib: A prescription medication marketed as Litfulo that inhibits JAK3 and members of the TEC kinase family. It is FDA approved for the treatment of severe alopecia areata in adults and adolescents 12 years of age and older.

Robotic FUE: Follicular unit excision performed with the assistance of a robotic or automated system designed to help identify and remove follicular units from the donor area. Robotic assistance does not eliminate the importance of physician judgment, donor management, surgical planning and graft placement.

Rogaine: A brand name for topical minoxidil, an FDA approved treatment for certain forms of androgenetic alopecia in men and women.

Retrograde Alopecia: A pattern of progressive thinning that begins along the lower margins of the scalp, particularly the nape and areas around the ears, and extends upward into regions that might otherwise be considered donor hair. Retrograde alopecia can reduce the amount of stable hair available for transplantation and should be identified during donor evaluation.

Safe Donor Zone: The region of the scalp considered most likely to contain hair follicles that will remain resistant to progressive androgenetic alopecia over a patient’s lifetime. Careful identification of the safe donor zone is essential in hair transplantation because follicles harvested outside stable donor areas may later miniaturize or be lost.

SALT Score: Short for Severity of Alopecia Tool, a standardized system used to measure the percentage of scalp hair loss in alopecia areata. SALT scoring is commonly used in clinical practice and research to evaluate disease severity and treatment response.

Sapphire FUE: A marketing term generally used to describe FUE hair transplantation in which recipient sites are created with blades made from synthetic sapphire rather than conventional steel instruments. Sapphire FUE is not a distinct method of harvesting donor follicles, and use of sapphire blades alone does not determine the quality or outcome of a hair transplant.

Saw Palmetto: An herbal extract commonly marketed for prostate health and hair loss because of possible effects on androgen metabolism and 5 alpha reductase activity. Evidence supporting saw palmetto as a treatment for androgenetic alopecia is limited compared with established medical therapies.

Scalp Biopsy: Removal of a small sample of scalp tissue for microscopic examination. A scalp biopsy can help distinguish between different forms of hair loss, particularly when inflammatory or scarring alopecia is suspected.

Scalp Laxity: The degree of flexibility and mobility of the scalp. Scalp laxity is especially important when planning follicular unit transplantation, or FUT, because it can affect how much donor tissue can be safely removed and the tension placed on the surgical closure.

Scalp Micropigmentation (SMP): A cosmetic procedure in which pigment is deposited into the scalp to create the appearance of closely cropped hair follicles, greater density or reduced contrast between the scalp and existing hair. SMP does not create new hair or treat the underlying cause of hair loss.

Scalp Reduction: A largely historical surgical hair restoration procedure in which areas of bald scalp were surgically removed and the surrounding hair bearing scalp was stretched or advanced to reduce the size of the bald area. The procedure has largely been abandoned because of scarring, altered scalp anatomy and the development of modern hair transplantation techniques.

Scarring Alopecia: A group of hair loss disorders in which inflammation or injury permanently destroys hair follicles and replaces them with scar tissue. Also known as cicatricial alopecia, these conditions require timely diagnosis because hair lost after follicular destruction generally cannot regrow.

Sebaceous Gland: A small gland associated with the hair follicle that produces sebum, an oily substance that helps lubricate the skin and hair. Sebaceous glands are part of the pilosebaceous unit.

Seborrheic Dermatitis: A common inflammatory skin condition that can cause redness, itching, flaking and greasy scale on the scalp. It does not typically cause permanent hair loss, although significant inflammation and scratching may contribute to temporary shedding.

Secondary Cicatricial Alopecia: Permanent hair loss caused when follicles are destroyed as a consequence of an external injury, infection, burn, radiation, tumor or other process rather than being the primary target of an inflammatory hair disorder.

Shedding: The release of hairs from the scalp as part of the normal hair cycle or as a result of a hair loss disorder. Increased shedding can occur with conditions such as telogen effluvium, alopecia areata, medication changes or during the early stages of some hair loss treatments.

Shock Loss: Temporary or sometimes permanent shedding of existing hair following hair transplant surgery or other significant scalp trauma. Shock loss most commonly affects vulnerable miniaturized hairs near the treated area, although transplanted hairs normally shed temporarily before beginning a new growth cycle.

Sinclair Scale: A clinical classification system used to describe the severity and progression of female pattern hair loss, particularly widening of the central part and decreasing hair density over the top of the scalp.

Spironolactone: A prescription potassium sparing diuretic with antiandrogen effects. It is commonly prescribed off label for selected women with female pattern hair loss, particularly when increased androgen activity is suspected or present.

Stem Cell Therapy: A broad term used for experimental approaches intended to regenerate, stimulate or replace cells involved in hair follicle growth. Stem cell based treatments are actively researched and heavily marketed for hair loss, but no stem cell therapy is currently an established FDA approved treatment for androgenetic alopecia.

Strip Surgery: A common term for follicular unit transplantation, or FUT, in which a narrow strip of hair bearing scalp is surgically removed from the donor area and dissected under magnification into individual follicular unit grafts for transplantation. The donor incision is closed surgically, leaving a linear scar.

Telogen: The resting phase of the hair growth cycle that follows catagen. At the end of telogen, the resting hair is eventually shed as the follicle prepares to begin a new anagen growth phase.

Telogen Effluvium: A form of diffuse hair shedding that occurs when an unusually large number of follicles enter the telogen phase at approximately the same time. Triggers may include illness, surgery, childbirth, significant psychological or physical stress, nutritional deficiency, rapid weight loss and certain medications. Acute telogen effluvium often resolves after the underlying trigger is corrected, although chronic forms can persist.

Temporal Recession: Progressive thinning or loss of hair at the temples. Temporal recession is a common feature of male androgenetic alopecia and may be one of its earliest visible signs.

Terminal Hair: A thick, pigmented hair that is longer and coarser than vellus hair. In androgenetic alopecia, terminal hairs progressively become finer and shorter as affected follicles undergo miniaturization.

Testosterone: A major androgen hormone produced primarily in the testes and, in smaller amounts, in the ovaries and adrenal glands. Testosterone can be converted by 5 alpha reductase into dihydrotestosterone, or DHT, which plays a central role in androgenetic alopecia in genetically susceptible follicles.

Thyroid Related Hair Loss: Hair shedding or changes in hair texture associated with abnormal thyroid function. Both hypothyroidism and hyperthyroidism can contribute to diffuse hair loss, and appropriate medical evaluation is important when thyroid disease is suspected.

Tinea Capitis: A fungal infection of the scalp and hair shafts that occurs most commonly in children. It can cause scaling, broken hairs, patches of hair loss and inflammation. Severe inflammatory infection can occasionally result in permanent scarring.

Topical Finasteride: Finasteride formulated for application directly to the scalp. It is used or compounded in some settings for androgenetic alopecia with the goal of reducing scalp DHT exposure while potentially limiting systemic exposure. Formulations vary, and topical finasteride is not FDA approved in the United States as a hair loss treatment.

Topical Minoxidil: Minoxidil applied directly to the scalp as a liquid or foam. It is FDA approved for certain forms of androgenetic alopecia in men and women and is available in several concentrations and formulations.

Traction Alopecia: Hair loss caused by repeated or prolonged tension on the hair follicles from tight hairstyles, extensions, braids, weaves or other forms of mechanical pulling. Early traction alopecia may be reversible, but long term tension can permanently damage follicles and produce scarring hair loss.

Transection: Accidental cutting or damage to a hair follicle or follicular unit during hair transplant harvesting or graft preparation. Excessive transection can reduce the number of viable grafts available for transplantation and may indicate poor surgical technique or inappropriate harvesting conditions.

Transplanted Hair: Hair follicles surgically moved from a donor area to a recipient area. When stable donor follicles are properly selected and transplanted, they generally retain the biological characteristics of the donor region.

Trichodynia: A sensation of scalp pain, burning, tenderness or discomfort sometimes reported by people experiencing hair loss or increased shedding. Trichodynia may occur with several scalp and hair loss conditions and does not by itself identify a specific diagnosis.

Trichoscopy: Magnified examination of the scalp and hair using a dermatoscope or digital imaging system. Trichoscopy can reveal patterns involving hair shaft diameter, follicular openings, scaling, inflammation and other features that help diagnose and monitor different forms of alopecia.

Trichotillomania: A condition characterized by recurrent pulling of one’s own hair, leading to noticeable hair loss. It is classified as a body focused repetitive behavior and can affect the scalp, eyebrows, eyelashes or other body hair.

Trichorrhexis Nodosa: A hair shaft abnormality in which weakened areas develop along the shaft, causing the hair to fracture easily. It can be inherited or acquired through physical, chemical or thermal damage.

Vascular Endothelial Growth Factor (VEGF): A signaling protein involved in the formation and maintenance of blood vessels. VEGF is produced within the hair follicle environment and is being studied for its role in supporting follicular growth and the vascular supply associated with the anagen phase.

Vellus Hair: A fine, short, lightly pigmented or nearly colorless hair. In androgenetic alopecia, progressive follicular miniaturization can cause thicker terminal hairs to become increasingly vellus like.

Vertex: The upper rear portion of the scalp, commonly referred to as the crown. The vertex is a frequent site of progressive thinning in male androgenetic alopecia and can also be affected in female pattern hair loss.

Vitamin D: A vitamin and hormone involved in numerous biological processes, including normal skin and hair follicle function. Vitamin D deficiency has been associated with several forms of hair loss, but supplementation has not been established as a hair growth treatment in people who are not deficient.