Hair Care Glossary: 100+ Terms Defined Simply

How to Use This Glossary

Terms are grouped thematically for practical use: hair structure first, then properties and conditions, then ingredient categories, then processes, then styling concepts. Cross-references point to related terms within the glossary or to our dedicated articles.

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Hair Structure

Cortex The innermost of the three main layers of the hair shaft (in hairs that lack a medulla), comprising approximately 45% of the hair's total volume. The cortex is where most of the hair's mechanical properties originate: tensile strength, elasticity, and resistance to breakage are all cortex-dependent. It is composed of densely packed keratin macrofibrils (intermediate filaments embedded in an amorphous keratin matrix) cross-linked by disulfide bonds. The cortex also contains melanin granules — the pigment cells that determine hair color.

Cuticle The outermost protective layer of the hair shaft, composed of 6–10 overlapping, scale-like cells (cuticle cells) that point from the root toward the tip, like roof tiles. Healthy cuticle cells lie flat, creating a smooth reflective surface and a functional barrier. Damaged cuticle cells lift, chip, or fracture — producing the rough texture, dullness, and porosity associated with chemically processed or mechanically stressed hair. The cuticle is coated by a mono-layer of 18-methyleicosanoic acid (18-MEA), a fatty acid covalently bound to the cuticle surface that gives healthy hair its hydrophobicity.

Medulla The optional central canal of the hair shaft, present in coarser, thicker hair fibers and absent or fragmented in fine hair. The medulla consists of loosely arranged, partially keratinized cells and air spaces. Its function is not fully established; it contributes to some optical properties of hair (the "shine" differential between coarse and fine hair). Most cosmetic treatments don't specifically target or affect the medulla.

18-MEA (18-Methyleicosanoic Acid) A branched, medium-chain fatty acid covalently bonded to the outer surface of each cuticle cell. 18-MEA is responsible for healthy hair's natural hydrophobicity — water beads off rather than immediately penetrating. Bleaching, alkaline chemical processing, and aggressive shampooing degrade the 18-MEA layer, making the cuticle surface hydrophilic (water-attracting), increasing porosity, and reducing smoothness. Many "bond repair" and conditioning treatments aim to replace or supplement this lipid layer.

Disulfide Bond A covalent chemical bond (S–S) formed between cysteine amino acid residues in adjacent keratin chains within the cortex. Disulfide bonds are the primary structural cross-links that give hair its strength and hold its shape. They are permanent under normal conditions but can be broken chemically (by alkaline reducing agents in perms and relaxers) or thermally (at very high heat). Oxidative coloring partially disrupts them. Bond-repair products (maleic acid derivatives, bis-aminopropyl diglycol dimaleate — "olaplex-type" actives) claim to restore broken disulfide bonds.

Hydrogen Bond A weaker electrostatic interaction between polar groups (–NH and –CO) in keratin chains. Hydrogen bonds are the reason hair styling works: heat and water temporarily break hydrogen bonds, allowing the hair to be reshaped; cooling or drying re-forms them in the new position. Blow-drying, curling, and flat-ironing all operate via this mechanism. Unlike disulfide bonds, hydrogen bonds break reversibly with water — which is why styled hair reverts in rain or humidity.

Salt Bond (Ionic Bond) An electrostatic attraction between positively charged amino acid side chains and negatively charged side chains in keratin. Like hydrogen bonds, salt bonds are pH-sensitive and disrupt with extreme acid or alkaline exposure. They are disrupted temporarily during shampooing (slightly alkaline scalp pH) and re-formed as the hair dries and pH normalizes.

Intercellular Cement (Cell Membrane Complex) The lipid-rich material between cortical cells and between cuticle cells that acts as structural glue and a diffusion barrier. The intercellular cement contains ceramides, cholesterol, fatty acids, and protein. Chemical processing — particularly bleaching — degrades intercellular cement, making hair feel "mushy" when wet and prone to splitting at the cortex level. Ceramide treatments aim to replenish this lipid matrix.

Keratin The primary structural protein of hair (and nails, skin surface, horn). Hair keratin is a hard alpha-keratin — tightly coiled protein chains (alpha helices) that form microfibrils, macrofibrils, and ultimately the cortex architecture. It is particularly rich in the sulfur-containing amino acid cysteine (creating disulfide cross-links) and has high tensile strength. "Keratin treatment" in the salon context refers to a smoothing process that applies hydrolyzed keratin and a sealing agent (often formaldehyde or a safer alternative) to temporarily fill surface defects — it is a conditioning process, not true structural keratin replacement.

Melanin The pigment responsible for hair color, synthesized in melanocyte cells in the hair follicle and deposited as granules in the cortex during hair growth. Two types: eumelanin (brown to black) and pheomelanin (red to yellow). Bleaching oxidizes and destroys melanin (making hair lighter); oxidative coloring simultaneously bleaches existing melanin and deposits new synthetic pigment.

Follicle The tube-like skin organ from which each hair fiber grows. The follicle extends from the scalp surface down into the dermis, terminating at the dermal papilla — the structure that supplies blood and signals to the hair matrix cells that produce the fiber. Hair growth phases (anagen, catagen, telogen — see below) occur within the follicle.

Dermal Papilla The highly vascular connective tissue structure at the base of the hair follicle. It supplies nutrients and growth signals to the hair matrix cells (keratinocytes) that divide rapidly to produce the hair fiber. The dermal papilla is the primary target of follicular-level treatments like minoxidil and low-level laser therapy — they work by improving perfusion and metabolic activity of the papilla.

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Hair Growth Phases

Anagen (Growth Phase) The active growth phase during which the hair matrix cells are actively dividing and the hair fiber is being produced. Scalp hair anagen lasts 2–7 years (genetically determined), during which the fiber grows approximately 1 cm per month. Approximately 85–90% of scalp hairs are in anagen at any given time.

Catagen (Transition Phase) A short, transitional phase (approximately 2–3 weeks) during which hair matrix cell division ceases, the follicle shortens, and the hair shaft detaches from the dermal papilla. Only ~1–2% of hairs are in catagen at a given time.

Telogen (Resting Phase) The quiescent phase (approximately 3 months) during which the old, detached hair ("club hair") is retained in the follicle while a new anagen hair begins growing beneath it. Telogen hairs are shed naturally or pushed out by the emerging new hair. Approximately 10–15% of scalp hairs are in telogen at any time. Normal daily shedding (50–100 hairs) is primarily telogen hairs.

Telogen Effluvium Diffuse, temporary hair shedding caused by a physiological stress that triggers an abnormally large proportion of follicles to enter telogen simultaneously. Common triggers: significant illness, surgery, childbirth, rapid weight loss, extreme psychological stress, iron deficiency, thyroid dysfunction. The shed occurs 2–4 months after the triggering event. Most cases are self-resolving, but the cause should be identified and addressed.

Anagen Effluvium Sudden, diffuse hair loss during the anagen (growth) phase, most commonly caused by chemotherapy agents that interrupt rapid cell division in the hair matrix. Unlike telogen effluvium, anagen effluvium presents with rapid, extensive shedding that can affect most of the scalp.

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Hair Properties and Conditions

Porosity A measure of the hair shaft's ability to absorb and retain moisture and products. Determined primarily by the cuticle's condition: a flat, intact cuticle = low porosity (slow absorption, good retention); a raised, chipped, or absent cuticle = high porosity (rapid absorption, rapid moisture loss). Porosity can be measured scientifically via mercury intrusion porosimetry or by observing water absorption speed; the "float test" is a popular but unreliable consumer method. High porosity typically results from bleaching, coloring, heat damage, or aggressive mechanical handling. Low porosity is often a natural trait of very fine, healthy hair.

Elasticity The hair shaft's ability to stretch under tension and return to its original length when the tension is released. Healthy hair can stretch approximately 20–30% before breaking (dry); wet hair stretches further but is more fragile at extreme extension. Low elasticity (hair that stretches and breaks without returning) indicates cortex damage — depletion of protein and disrupted disulfide cross-links. Elasticity is the key indicator tested in professional hair assessments before chemical services.

Hygral Fatigue Damage caused by repeated cycles of water absorption (swelling) and drying (contraction). Each cycle temporarily expands the cortex by up to 18% in diameter; the repeated mechanical stress on the cuticle and cortex over time degrades structural integrity. Highly porous hair is most susceptible because it absorbs and loses water most rapidly. Consistent pre-wash oiling (particularly coconut oil, which partially blocks water uptake) reduces hygral fatigue.

Tensile Strength The maximum stress a hair fiber can withstand before breaking. Measured in grams-force per fiber in laboratory tensile testing. Chemical processing (especially repeated bleaching), heat damage, and protein depletion reduce tensile strength measurably.

Hair Density The number of hair follicles per unit area of scalp — distinct from individual strand thickness. A person can have high density (many hairs) but fine individual strands, or low density with coarse individual strands. Density determines the visual "fullness" of the hair. Hair loss conditions (androgenetic alopecia, alopecia areata) primarily affect density.

Hair Texture / Fiber Diameter The cross-sectional diameter of an individual hair fiber. Ranges from fine (<0.060 mm) to medium (0.060–0.090 mm) to coarse (>0.090 mm). Fiber diameter is genetically determined per follicle and is not permanently changeable by cosmetic products (though products can temporarily swell or smooth the cuticle, altering apparent diameter and feel). Androgenetic alopecia produces miniaturization — progressive reduction in fiber diameter per follicle over successive growth cycles.

Curl Pattern (Type System) A classification system for hair curvature, popularized in the 1990s (commonly attributed to Andre Walker), ranging from Type 1 (straight) through Type 2 (wavy), Type 3 (curly), and Type 4 (coily/kinky), with subtypes A–C within each indicating curl tightness. While widely used as a shorthand, the system is reductive: hair curvature results from follicle shape (oval follicles produce curved fibers) and doesn't capture porosity, density, or texture, which matter as much for product selection. Use it as a starting framework, not a definitive guide.

Split End (Trichoptilosis) Longitudinal splitting of the hair shaft at the tip, caused by cuticle and cortex mechanical fracture from friction, heat, chemical damage, or weathering. Split ends cannot be repaired by products — the fiber is fractured. Temporary cosmetic improvement is possible (conditioning agents smooth the split temporarily), but the only effective treatment is trimming. Regular trims prevent splits from propagating further up the shaft.

Breakage Transverse fracture of the hair shaft at any point along its length (as opposed to shedding, which involves detachment at the root). Breakage is caused by physical stress (aggressive brushing, tight hairstyles, friction on pillow), chemical damage (bleaching, over-processing), or nutritional deficiency that weakens the fiber structure. Breakage hairs have blunt or frayed ends; shed hairs have a visible white bulb at the root end.

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Ingredient Categories

Surfactant Surface-active agent — a molecule with a water-soluble (hydrophilic) head and an oil-soluble (hydrophobic) tail that emulsifies sebum and debris for rinsing. Classified as anionic (negatively charged — most shampoo detergents), cationic (positively charged — conditioner agents), amphoteric (zwitterionic — mild co-surfactants like cocamidopropyl betaine), and nonionic (uncharged — very mild, glucoside-based). See also: sulfate-free shampoo guide.

Anionic Surfactant A surfactant with a negatively charged hydrophilic head. The most effective sebum-removing surfactants belong to this class. Sodium lauryl sulfate (SLS), sodium laureth sulfate (SLES), and sodium lauroyl sarcosinate are all anionic. Most cause some degree of cuticle swelling and lipid stripping; potency and irritancy vary by specific compound.

Cationic Surfactant / Conditioning Agent A positively charged surfactant. Hair carries a net negative charge (amplified by damage), so cationic agents deposit electrostatically and persistently on the hair surface. Behentrimonium chloride (BTAC), cetrimonium chloride, and cetrimonium bromide are the primary cationic conditioners in commercial products. They reduce friction, improve detangling, and impart softness.

Amphoteric Surfactant A surfactant that carries both positive and negative charges depending on pH. Cocamidopropyl betaine (CAPB) is the most common; it functions as a mild co-surfactant in sulfate-free formulas and as a foam booster in sulfate-containing ones. Lower irritancy potential than anionic sulfates.

Humectant An ingredient that attracts and holds water molecules from the environment (hygroscopic) or from the dermis. In hair care: glycerin, hyaluronic acid, panthenol, aloe vera, sorbitol, and honey are humectants. They help maintain hydration in the hair shaft but can worsen frizz in very high humidity if the cuticle is raised and cannot contain the attracted moisture. Balance humectants with emollients and occlusives.

Emollient An ingredient that softens and smooths the hair surface by filling cuticle gaps, reducing friction, and improving flexibility. Oils, butters, and fatty alcohols (cetyl alcohol, stearyl alcohol) are emollients. Distinguished from humectants (which attract water) and occlusives (which prevent water escape). Emollients improve the feel and manageability of hair without necessarily adding moisture.

Occlusive An ingredient that forms a physical barrier on the hair surface to prevent moisture evaporation. Dimethicone, mineral oil, petrolatum, and beeswax are occlusives. Most effective on high-porosity hair that loses moisture rapidly; can feel heavy or cause buildup on healthy, low-porosity hair. Used as a "seal" in the LOC (Liquid-Oil-Cream) and LCO methods popular in curly hair routines.

Protein (Hydrolyzed Protein) Proteins derived from various sources (keratin, collagen, wheat, silk, oat, quinoa) that have been enzymatically or chemically hydrolyzed (broken into smaller fragments). Small hydrolyzed peptides can penetrate the cuticle and temporarily fill cortex gaps, providing a strengthening and smoothing effect. Larger protein molecules coat the surface. Protein treatments can cause "protein overload" in hair that is healthy or already high in protein — manifesting as stiffness and brittleness. Balance protein treatments with moisturizing conditioning.

Ceramide A family of lipid molecules (sphingosine + fatty acid) that form a major component of the intercellular cement between cortex and cuticle cells. Depletion of ceramides — from bleaching, alkaline processing, or aggressive surfactants — disrupts the cuticle's structural integrity. Ceramide-containing hair treatments aim to replenish this lipid matrix, improving cuticle adhesion and moisture barrier function.

Panthenol (Pro-Vitamin B5) A water-soluble alcohol that converts to pantothenic acid (vitamin B5) in biological tissue. In hair care, panthenol's primary functions are as a humectant (it absorbs and holds water) and as a fiber-swelling agent that temporarily thickens fine hair. It penetrates the cortex, attracting moisture. Well-tolerated, commonly used in conditioners and leave-in treatments.

Niacinamide (Vitamin B3) In skin care, niacinamide is a highly multi-functional ingredient (barrier support, anti-inflammatory, pigmentation reduction). In hair care, its evidence base is thinner but growing; it may support scalp barrier function and follicular health. Included in some scalp serums targeting hair density and DHT-related inflammation.

Fatty Alcohol Not an irritant alcohol (like isopropyl or ethanol) — fatty alcohols are long-chain waxy molecules (cetyl alcohol, stearyl alcohol, cetearyl alcohol) that function as emollients, emulsifiers, and texture agents in conditioners. They are gentle, non-drying, and provide the thick, creamy texture of conditioners. The "alcohol" in the name confuses many consumers into avoiding them unnecessarily.

Silicone See full entry in the silicones in hair products guide. In brief: synthetic polymer conditioning agents that coat the hair cuticle to reduce friction, block humidity, and impart shine. Water-insoluble types (dimethicone) require sulfate-containing cleansers for removal; water-soluble types (dimethicone copolyol) are compatible with gentler routines.

Benzyl Alcohol A short-chain aromatic alcohol used as a preservative and solvent in hair products. At the concentrations used in cosmetics (<1%), it is safe and does not cause the drying associated with isopropyl or ethanol. However, those with fragrance sensitivity may react to it (it is also used as a fragrance ingredient).

Behentrimonium Chloride (BTAC) The primary cationic conditioning agent in most modern leave-in conditioners and rinse-out conditioners. BTAC is a quaternary ammonium compound (quats) that deposits on hair electrostatically — particularly on negatively charged, damaged zones — providing significant detangling, friction reduction, and softness. It is considered milder than its predecessor, cetrimonium chloride, in most formulations.

Cetyl Alcohol / Stearyl Alcohol / Cetearyl Alcohol Fatty alcohols (not drying alcohols) used as emulsion stabilizers, emollients, and thickeners in conditioners. They give conditioners their creamy consistency and contribute to slip and softness. Cetearyl alcohol is a blend of cetyl (C16) and stearyl (C18) alcohols. Safe for all hair types.

EDTA (Disodium EDTA) A chelating agent added to hair products to bind and inactivate calcium and magnesium ions from hard water, which otherwise interfere with surfactant performance and deposit on the hair as mineral scale. Effective at improving wash performance in hard-water areas.

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Chemical Processes

Oxidative Hair Color A hair coloring method using an alkaline developer (typically hydrogen peroxide) to open the cuticle, oxidize and bleach existing melanin, and simultaneously oxidize colorless dye precursors (typically para-phenylenediamine derivatives, PPD) into large, colored molecules trapped within the cortex. Damage is proportional to developer concentration (volume) and processing time. Requires periodic retouching as new growth emerges.

Bleaching An oxidative process using hydrogen peroxide (typically at higher volumes than color) with an alkaline boosting powder (persulfate salts) to maximally oxidize and destroy melanin in the cortex. Bleaching degrades the 18-MEA lipid layer, depletes cortex proteins, disrupts disulfide bonds, and removes intercellular lipids — the most structurally damaging common hair service.

Chemical Relaxing A permanent process that breaks disulfide bonds in the cortex using highly alkaline reducing agents (sodium hydroxide — "lye relaxers," or guanidine carbonate — "no-lye relaxers"), applies tension to straighten the fiber, then oxidizes the bonds in their new configuration. Leaves hair significantly more fragile and porous; requires careful conditioning maintenance.

Perming A permanent curling or waving process using a reducing agent (thioglycolate) to break disulfide bonds, applying rods to configure the new curl shape, then re-oxidizing bonds with a neutralizer (hydrogen peroxide or sodium bromate) to fix the curl. Similar structural disruption to relaxing but in the opposite direction.

Keratin Smoothing Treatment A semi-permanent smoothing service using hydrolyzed keratin applied to the hair, then heat-sealed under a flat iron. The heat fuses the keratin into the cuticle temporarily, creating smoothness that lasts weeks to months. Most professional formulas previously contained formaldehyde as a cross-linking agent; many now use safer alternatives (glyoxylic acid, glutaraldehyde). Results fade gradually as the hair is washed.

Clarifying The process of removing product buildup, mineral deposits, and excess oil from the hair and scalp using a stronger-than-normal surfactant (clarifying shampoo). Recommended periodically (every 2–4 weeks for product-heavy routines) when buildup causes dullness, heaviness, or reduced product responsiveness. Most clarifying shampoos use SLES or a blend of anionic surfactants and often contain chelating agents (EDTA) for mineral removal.

Chelating Wash A targeted clarifying process specifically for removing hard water mineral deposits (calcium, magnesium) using chelating agents (EDTA, citric acid, phytic acid). Particularly valuable in high-calcium water areas where deposits cause hair to feel gritty, look dull, and resist coloring.

Co-Washing (Conditioner-Only Washing) Washing the hair using conditioner instead of shampoo, relying on conditioner's mild surfactant content (typically cocamidopropyl betaine from the formulation) to cleanse while conditioning simultaneously. Most appropriate for highly porous, coily hair types that require maximum moisture retention. Insufficient for removing heavy styling products or silicone buildup; periodic shampoo or clarifying washes are still necessary.

Toning The application of a semi-permanent or demi-permanent color product to neutralize or shift unwanted warmth (brassiness, yellow, orange) from bleached or highlighted hair. Toners deposit pigment without lifting (no peroxide developer above 10 volume), fade over 4–6 weeks, and require no new growth maintenance.

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Styling Concepts

Hold Factor A product attribute describing how strongly a styling product fixes the hair in its set position. Products are typically rated "light/flexible hold," "medium hold," "strong/firm hold," or "extra hold." Provided by film-forming polymers (PVP, acrylates) that cross-link into a rigid film as the product dries.

Cast The stiff, crunchy coating that some curl-defining products (gels, custards with high polymer content) create around hair as they dry. Intended to be "scrunched out" once fully dry, leaving defined curls without crunch. Cast formation indicates the product's polymer is doing its job; it should not be touched while wet (disrupts the gel matrix and causes frizz).

Crunch The tactile stiffness of dried gel or polymer film on the hair. Used positively in the curly hair community: "good crunch" means strong cast that will scrunch out to reveal soft, defined curls; "no crunch" means the gel hasn't fully set.

Frizz Disruption of the smooth cuticle surface, causing individual fibers to swell, roughen, and separate from their pattern. Caused by humidity (water vapor swells the cortex), high porosity (rough cuticle catches on adjacent fibers), or lack of definition (no product to group fibers). Managed by: smoothing the cuticle (silicones, heavy oils), humidity-blocking occlusives, and grouping fibers into defined units (gels, custards).

Plopping A curly-hair drying technique where damp hair is placed onto a t-shirt or microfiber towel laid flat, then the edges are twisted or secured around the head to "plop" the curls into their natural formation while absorbing excess water without causing frizz from rubbing. Reduces dry time and improves curl definition.

Diffusing Drying hair with a hair dryer attachment (the diffuser — a broad, finger-like attachment) that disperses airflow widely and reduces direct air pressure, allowing curls to dry in their natural clumped pattern rather than being blown apart by direct airflow. Lower heat settings are recommended.

LOC / LCO Method Layering systems for moisture retention in curly and coily hair. LOC = Liquid (water or water-based product) → Oil → Cream. LCO = Liquid → Cream → Oil. The idea is to layer moisture (liquid), lock it in (oil as an occlusive), then seal and define (cream). Which order works better is hair-type dependent: denser, coarser hair benefits from LCO (cream before oil helps moisture penetrate); finer hair may prefer LOC.

Sealing Applying an occlusive product (oil, butter, silicone) as the final step after moisturizing, to physically trap moisture inside the hair shaft. Particularly important for high-porosity hair that loses moisture quickly.

Slip The lubricity or glide provided by a conditioner or detangling product. High slip means the fingers or a comb pass through the hair smoothly with minimal resistance and breakage. Provided by cationic surfactants (BTAC), fatty alcohols, silicones, and oils.

Protein Overload A condition where excessive protein treatment has made hair feel hard, stiff, brittle, and prone to snapping. Hair feels rough even when wet, doesn't stretch before breaking, and may feel "straw-like." Resolved by moisturizing treatments and temporarily avoiding protein products. Most common in fine hair types or in routines involving multiple protein-containing products.

Moisture Overload The opposite of protein overload: hair that is extremely soft, limp, and lacks any elasticity — it stretches and does not return. Caused by excessive moisturizing without protein balance. Resolved by protein treatments. Healthy hair requires a balance between moisture and protein.

Hygroscopic Capable of absorbing moisture from the surrounding air. Humectants are hygroscopic. In high humidity, hygroscopic products can attract excessive moisture, causing hair to swell unevenly and frizz. In very low humidity, hygroscopic humectants may draw moisture out of the hair instead of from the environment — a problem in arid climates.

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Scalp Terms

Sebum The waxy, oily secretion produced by sebaceous glands adjacent to each hair follicle. Sebum is a complex mixture of triglycerides, wax esters, squalene, and free fatty acids. It lubricates and conditions the scalp and upper part of the hair shaft (it doesn't travel far down a curled fiber, explaining why curly hair is naturally drier). Overproduction causes greasy hair; underproduction causes dry scalp.

Sebaceous Gland The oil-secreting gland attached to each hair follicle that produces sebum. Sebaceous activity is influenced by androgens (explaining why hormonal changes during puberty, pregnancy, and menopause affect scalp oiliness) and can be temporarily affected by aggressive cleansing.

Dandruff (Pityriasis Capitis) A common, non-contagious scalp condition characterized by visible white or yellowish flakes shed from the scalp, often accompanied by itching. Associated with the yeast Malassezia globosa, which metabolizes scalp sebum and produces oleic acid — a fatty acid that triggers inflammatory desquamation in susceptible individuals. Treated with antifungal agents (ketoconazole, selenium sulfide, zinc pyrithione, piroctone olamine) or keratolytics (salicylic acid, coal tar).

Seborrheic Dermatitis A more severe inflammatory form of dandruff involving greater scalp redness, greasiness, and thicker adherent flakes. Can extend to the face (eyebrows, nasolabial folds, ears). A chronic condition managed, not cured, with antifungal shampoos and, in more severe cases, topical corticosteroids or calcineurin inhibitors.

Scalp Microbiome The community of microorganisms (bacteria, fungi) that colonize the scalp surface. Malassezia species are the dominant fungi; bacterial communities include Staphylococcus, Cutibacterium (formerly Propionibacterium), and others. An imbalanced scalp microbiome is implicated in dandruff, seborrheic dermatitis, and possibly some forms of folliculitis. Extremely aggressive or frequent cleansing can disrupt the microbiome.

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Product Format Terms

Rinse-Out Conditioner A conditioner applied post-shampoo and rinsed out after 1–5 minutes. Deposits cationic conditioning agents and emollients on the cuticle, improving slip, detangling, and softness. Does not need to "penetrate" — it works on the surface.

Deep Conditioner / Hair Mask A high-concentration conditioner designed for longer application (15–30 minutes or more, often with heat) to deliver more intensive conditioning. Typically higher in cationic agents, humectants, and proteins/ceramides. Used weekly to monthly depending on hair condition.

Leave-In Conditioner A lightweight conditioner applied to damp hair that is not rinsed out. Provides ongoing moisture, detangling, and protection without the weight of a rinse-out formula. Forms the "liquid" or "cream" layer in LOC/LCO methods.

Serum A concentrated, often silicone-rich product applied to dry or damp hair for surface-level smoothing, shine, frizz control, and heat protection. Usually the last step in a styling routine. High-viscosity serums (containing dimethicone or phenyl trimethicone) provide the most slip and shine.

Hair Oil (as a product format) A standalone oil or oil blend for pre-wash treatment, scalp care, or finishing. Differs from a "serum" in that it contains primarily or exclusively oils (or silicones marketed as oils) without the aqueous, surfactant, and polymer components of a full conditioner formulation. See the hair oils guide.

Clarifying Shampoo A shampoo formulated specifically for periodic deep cleansing, containing higher concentrations of stronger surfactants and often chelating agents. Used to remove product buildup, silicone deposits, and mineral scale. Not intended for daily use.

Bond Repair Treatment A category of products (professional and at-home) that claim to repair broken chemical bonds in the cortex, most prominently disulfide bonds disrupted by bleaching and chemical processing. The best-evidenced active chemistry uses maleic acid derivatives or bis-aminopropyl diglycol dimaleate (BAPD, the olaplex-type molecule). Applied during or after chemical services or used as a standalone treatment.