Glossary · 104 terms

Learn to readthe label.

The words on the back of a product no longer need to feel foreign. From skin science to active ingredients, from the microbiome to hair structure, every term in plain language.

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Acid mantle: A thin, mildly acidic film covering the skin's surface, with an average pH of around 5. It helps barrier enzymes do their work and helps beneficial bacteria settle in; harsh soaps can upset this balance.
Acne: A common skin condition that shows up as blackheads, whiteheads, pimples and cysts when the pores connected to the oil glands become clogged and inflamed. Hormones, sebum, disrupted cell turnover and microbiome imbalance all play a role together.
AHA (alpha hydroxy acids): The umbrella name for water-soluble acids such as glycolic acid, lactic acid and mandelic acid. They loosen the bonds between dead cells to renew the skin's surface, giving it radiance and smoothness. They can increase sun sensitivity, so sunscreen is a must while using them.
Allantoin: A soothing, softening skincare ingredient with a low risk of irritation. It is often used for sensitive and irritated skin.
Anagen: The active growth phase of a hair strand. Most of the hairs on your scalp are in this phase, and it can last for years; the length of the anagen phase determines how long your hair can grow.
Antioxidant: Substances that help neutralize free radicals generated by the sun, pollution and smoking. Vitamin C, vitamin E and niacinamide are among the antioxidants most often used on the skin.
Atopic dermatitis (eczema): A chronic skin condition that flares up and calms down, marked by dryness, intense itching and red patches. A weakened barrier and microbiome imbalance play an important role; see a dermatologist for diagnosis and treatment.
Azelaic acid: A relatively gentle active ingredient used for breakouts, post-acne marks and redness linked to rosacea. It also helps with uneven tone; it is generally considered safe during pregnancy, but you should check with your doctor.

Bakuchiol: An active ingredient reported to show some retinol-like effects. Because it is thought to be less irritating than retinoids, it can be a preferred alternative for sensitive skin.
Barrier (skin barrier): The protective function of the skin's outermost layer that prevents water loss and keeps germs, pollution and irritants out. Dead cells are the bricks, and the ceramides and lipids between them are the mortar; the microbiome is also a living part of this shield.
BHA (beta hydroxy acid): In skincare it is best known as salicylic acid. Because it is oil-soluble, it can get inside the pore and help clear clogs; it is a favorite for oily and acne-prone skin.
Biofilm: A structure in which microorganisms attach to a surface and live as a community inside a sticky layer they produce themselves. On the skin, balanced communities formed by beneficial bacteria can be protective, while unbalanced biofilms have been linked to certain skin problems.
Blue light: High-energy visible light found in sunlight and, at lower levels, in screens. It has been reported to contribute to pigmentation, especially in people with darker skin tones; tinted sunscreens containing iron oxides can offer protection.
Broad spectrum: A label indicating that a sunscreen protects against both UVB and UVA rays. The SPF value alone does not tell you about UVA protection.

Catagen: A short transition period of a few weeks in which hair moves from the growth phase to the resting phase. During this phase the hair root shrinks and receives less nourishment.
Ceramide: Lipid molecules that make up roughly half of the lipids forming the "mortar" of the skin barrier. When they decrease, dryness and sensitivity increase; applied together with cholesterol and fatty acids, they help support the barrier.
Chemical peel: A procedure in which a specific acid solution is applied to the skin to remove the upper layers in a controlled way. There are superficial, medium and deep types; it is used for dark spots, acne scars and fine lines and should be performed by a professional.
Collagen: The main structural protein that makes up most of the dermis and gives the skin plumpness and firmness. It decreases with age, sun exposure and menopause; collagen in creams does not penetrate deeply and works as a moisture binder.
Comedone: The precursor of a pimple, formed when sebum and dead cells clog a pore. If the pore opening is open it is called a blackhead; if it is closed, a whitehead.
Commensal: Microorganisms that live on our skin or in our gut without causing harm, and often while providing benefits. They contribute to balance by leaving no room for harmful microbes and by training the immune system.
Contact dermatitis: Skin inflammation that appears after contact with a substance. It can be irritant or allergic and shows up as redness, itching and blisters. The cause of the allergic type can be investigated with a patch test.
Cortex: The middle and thickest layer of the hair strand. This is where the keratin fibers and pigments that give hair its strength, elasticity and color are found.
Cuticle: The outermost layer of the hair strand, made of cells that overlap like roof shingles. When the cuticle lies smooth and closed, hair looks shiny and sleek; when it is damaged, hair turns dull and frizzy.

Dandruff: White or yellowish flakes shedding from the scalp. It is linked to scalp sebum, yeasts and a disrupted barrier balance; itching may come with it.
Dehydrated skin: A temporary state in which the skin lacks water rather than oil. It can occur even in oily skin and shows up as tightness, dullness and more visible fine lines.
Dermis: The layer beneath the epidermis, rich in collagen and elastin fibers and home to blood vessels, nerves, hair roots and oil glands. The skin's firmness and elasticity depend largely on the dermis.
Double cleansing: A method in which you first dissolve makeup and sunscreen with an oil-based cleanser, then wash with a gentle water-based cleanser. It is especially helpful in the evening for those who wear makeup.
Dysbiosis: A disruption of the microbiome's balance: fewer beneficial bacteria, lower diversity, or certain species becoming overly dominant. It has been linked to conditions such as acne, atopic dermatitis and dandruff.

Elastin: A flexible protein fiber in the dermis that allows the skin to snap back after being stretched. It is damaged by age and sun; it is one of the causes of sagging.
Emollient: Oily ingredients that fill the gaps between dead cells, making the skin feel soft and smooth. Squalane, shea butter and fatty esters are examples.
Epidermis: The outermost layer of the skin, the one we can see. It renews itself roughly every four weeks; its topmost layer, the stratum corneum, carries the main load of the barrier.
Erythema: The medical term for redness of the skin. It can have many causes, such as sunburn, irritation, inflammation or rosacea.
Estrogen: One of the female hormones. It helps maintain collagen, moisture and thickness in the skin, and helps keep hair in its growth phase; its decline at menopause leads to noticeable changes in skin and hair.
Exfoliation: Removing dead cells from the skin's surface. It can be done chemically with acids and enzymes or physically with grainy products; overdoing it damages the barrier.
Exposome: The sum of all the external factors a person is exposed to over a lifetime: sun, air pollution, smoking, diet, sleep, stress and more. A significant part of skin aging is thought to be related to the exposome.

Ferment: An ingredient obtained when microorganisms ferment a substance. The small molecules, acids and peptides formed during fermentation are used in skincare products to support moisture and the barrier.
Fibroblast: Cells in the dermis that produce collagen, elastin and hyaluronic acid. Actives such as retinoids and vitamin C can help support the work of these cells.
Filaggrin: A protein that holds keratin fibers together in the skin's upper layer and, when broken down, becomes a source of natural moisturizing factor. A deficiency is linked to dry skin and atopic dermatitis.
Fine line: The first, superficial wrinkles that appear on the skin. They are usually related to dehydration, facial expressions and sun; their appearance can be reduced with moisturizing and retinoids.

Glycerin: One of the cheapest yet best-proven humectants, drawing water from the air and deeper layers up to the skin's surface. It suits almost every skin type.
Glycolic acid: The AHA with the smallest molecular size. It penetrates the skin quickly and deeply and provides strong resurfacing; that is why it can irritate sensitive and dry skin.
Gut-skin axis: A concept describing the two-way relationship between the gut microbiome and skin health. Diet, gut flora and the immune system are thought to influence the skin's condition, and this is being actively researched.

Hair follicle: The tube-shaped structure in the skin from which a hair strand emerges and grows. The hair's growth cycle, thickness and shape depend on the follicle.
Humectant: Ingredients that hydrate the skin by attracting water. Glycerin, hyaluronic acid, urea and panthenol are examples; their effect increases when a cream is applied on top.
Hyaluronic acid: A humectant found naturally in the skin that can bind many times its own weight in water. Different molecular sizes provide moisture in different layers of the skin; the skin's own hyaluronic acid decreases with age.
Hyperpigmentation: When certain areas of the skin become darker than the surrounding skin. Sun spots, melasma and post-acne marks are the most common types.

Inflammation: The immune system's inflammatory response to irritation, infection or damage. It shows up as redness, warmth, swelling and pain; long-term, low-grade inflammation can contribute to skin aging.
Irritation: Stinging, burning, redness and flaking caused when a substance or factor directly damages the skin. Unlike an allergy, it does not require immune memory; reducing the dose and frequency is often enough.

Keratin: The tough structural protein that makes up the skin's upper layer, hair and nails. In hair, it is the main material of the cuticle and cortex.
Keratinocyte: The most common cell in the epidermis. It is born in the lower layer, fills with keratin as it moves up, and finally becomes a flattened dead cell on the surface: the "brick" of the barrier.

Lactic acid: A gentle AHA with a larger molecular size. It removes dead cells and, because it is part of the natural moisturizing factor, also holds moisture; it is a good starter acid for dry and slightly sensitive skin.
Laser: A medical device that acts on target tissue in the skin with light concentrated at a specific wavelength. It is used to treat dark spots, broken capillaries, scars and wrinkles; it must always be performed by a qualified physician.
Lipid: The general name for fats and fat-like molecules. In the skin, ceramides, cholesterol and free fatty acids are the key lipids that form the barrier's "mortar."
Lysate: An ingredient obtained by breaking down probiotic bacteria; it contains no live bacteria. It holds cell fragments and intracellular substances and is used in skincare products to help support the barrier and microbiome balance.

Melanin: The pigment that gives color to the skin, hair and eyes. It partly protects the skin against UV rays; excessive or uneven production leads to dark spots.
Melanocyte: Cells in the lower layer of the epidermis that produce melanin. Sun, hormones and inflammation can stimulate these cells and trigger dark spots.
Melasma: A pigmentation disorder that appears as symmetrical brown patches, especially on the cheeks, forehead and upper lip. It is more common in women, during pregnancy and with hormone use; sun and heat trigger it.
Micellar water: A light cleansing water containing tiny clusters of surfactants (micelles) that trap oil and dirt. It gently dissolves makeup; it is a good idea to rinse afterward or do a second cleanse.
Microbiome: All the microorganisms living in a given environment, together with their genes and the substances they produce. The skin microbiome is an important part of the barrier, immunity and the skin's overall balance.
Microbiota: The community of microorganisms itself (bacteria, fungi, viruses and others) living in a given environment. It is often used interchangeably with microbiome; microbiome is the broader concept.
Microneedling: A procedure that aims to stimulate the repair process and collagen production by creating controlled micro-channels in the skin with very fine needles. It is used for acne scars and uneven texture; it requires hygiene and expertise.
Mineral filter: Inorganic sunscreen filters such as zinc oxide and titanium dioxide that reflect and absorb UV rays. They are better tolerated by sensitive skin; sometimes they can leave a white cast.
Minoxidil: A medication applied to the scalp that helps keep hair follicles in the growth phase in cases of hair loss. It is used for female pattern hair loss; it requires consistent use and should be started on a doctor's recommendation.
Moisturizer: A skincare product that combines humectant, emollient and occlusive ingredients to increase the skin's hydration and support the barrier. Every skin type needs one, oily skin included.

Natural moisturizing factor (NMF): A mix of small water-binding molecules such as amino acids, urea and lactate found inside the cells of the skin's upper layer. It is the skin's own moisturizer; when it decreases, dryness and flaking increase.
Niacinamide: A form of vitamin B3. It helps strengthen the barrier, balance sebum and reduce the look of redness and dark spots; it is a versatile active that is usually well tolerated.

Occlusive: Ingredients that form a thin film on the skin to prevent water from evaporating. Petrolatum, waxes and some silicones are examples; they reduce moisture loss, especially in very dry skin.
Oxidative stress: A state in which free radicals overwhelm the body's antioxidant defenses. In the skin it can speed up collagen loss, pigmentation and premature aging.

PA: A system that shows a sunscreen's level of UVA protection with plus signs. PA+ means the lowest and PA++++ the highest UVA protection.
Panthenol: Provitamin B5. In the skin it is converted to pantothenic acid; it holds moisture, soothes and supports barrier repair. It is also used in hair care for moisture and shine.
Patch test: A medical test used when allergic contact dermatitis is suspected, in which possible allergens are applied to the back with tape and evaluated after 48–72 hours. At home, trying a new product behind the ear for a few days offers a simple preliminary check.
Peptide: Short chains of amino acids. Some peptides are thought to signal the skin to repair and produce collagen; they are used as supportive ingredients in care for firmness and fine lines.
Petrolatum (petroleum jelly): One of the occlusive ingredients that most effectively reduces water loss. It is often used on very dry, cracked areas and to protect the barrier after procedures; on oily skin prone to clogged pores, it should be used carefully on the face.
pH: A scale from 0 to 14 that shows how acidic or alkaline an environment is. A healthy skin surface is mildly acidic (about 4.5–5.5); most soaps are alkaline, so they can upset this balance.
PHA (polyhydroxy acids): The gentlest group of acids, with large molecules, such as gluconolactone and lactobionic acid. They provide slow, superficial renewal and hold moisture; they are suitable for sensitive skin.
Photoaging: Aging caused in the skin over the years by the sun, especially UVA rays: deep wrinkles, dark spots, sagging and thickened texture. It can be slowed down with regular sun protection.
Pore: The opening of a hair follicle and oil gland on the skin's surface. Pore size is largely genetic; sebum and clogging can make pores look more visible, but pores do not open and close.
Porosity: A hair strand's capacity to absorb and hold water and care products. With low porosity the cuticle is tightly closed and products struggle to work; with high porosity hair gets wet quickly but also dries quickly and turns frizzy.
Postbiotic: Preparations made of inanimate microorganisms and their components that may benefit health. Because they contain no live bacteria, they are attracting interest as a stable microbiome-support option in skincare products.
Prebiotic: Substances selectively used by beneficial microorganisms, thereby contributing to health. On the skin, they can help support microbiome balance by creating an environment that favors beneficial bacteria.
Probiotic: Live microorganisms that provide a health benefit when taken or applied in adequate amounts. Research is exploring whether they can help support the barrier and microbiome balance on the skin; in skincare products they appear as live or probiotic-derived ingredients.

Retinal (retinaldehyde): A vitamin A derivative one step more active than retinol. The skin converts it to retinoic acid by a shorter route; it can deliver results faster than retinol, and its irritation potential is also slightly higher.
Retinoid: The general name for vitamin A derivatives: retinol, retinal, adapalene, tretinoin and others. They regulate cell turnover and support collagen production; they are the group with the most evidence in acne and anti-aging care. They should not be used during pregnancy.
Retinol: The most common vitamin A derivative in skincare. In the skin it is converted first to retinal, then to the active form, retinoic acid; starting with a low dose and building up gradually is recommended.
Rosacea: A chronic skin condition marked by redness, visible blood vessels and pimple-like bumps on the cheeks, nose, forehead and chin. Heat, sun, spicy food and alcohol can trigger it; diagnosis and treatment should be done with a dermatologist.

Salicylic acid: The best-known BHA. It is oil-soluble, helps clear sebum and dead cells from inside the pores and calms inflammation. It is suitable for oily and acne-prone skin.
Sebaceous gland: A gland connected to hair follicles that produces sebum. These glands are concentrated on the face, scalp, chest and back; they are regulated by androgen hormones.
Sebum: The natural oil produced by the oil glands, made up of triglycerides, wax esters and squalene. It softens the skin and hair and reduces water loss; too much of it leads to shine and clogged pores.
Shampoo: A product containing surfactants used to remove sebum, dirt and product buildup from the scalp and hair. Its real target is the scalp; gentle shampoos close to the scalp's pH help protect the barrier and microbiome.
SPF (sun protection factor): A value showing how much protection a sunscreen provides, mainly against UVB rays. SPF 30 blocks about 97 percent and SPF 50 about 98 percent of UVB; applying enough and reapplying are essential for protection.
Squalane: A more stable form of squalene, which is found in the skin's own oil. It is a lightweight, non-greasy emollient with a low risk of irritation.
Stratum corneum: The thin, topmost layer of the epidermis, made of flattened dead cells and the lipids between them. It carries the main load of the skin barrier and is described with the brick-and-mortar model.
Sulfate: Strongly foaming surfactants such as sodium lauryl sulfate. They clean oil effectively but can strip too much of the barrier and moisture from sensitive or dry skin and from color-treated hair.

Telogen: The resting phase of hair. It lasts about three months, after which the strand falls out and a new one starts to grow in its place; shedding 50–100 hairs a day is a natural part of this cycle.
Telogen effluvium: Shedding that happens when a large number of hairs enter the resting phase at the same time, 2–3 months after events such as childbirth, high fever, sudden weight loss, severe stress or surgery. It is usually temporary; you can see a doctor to look into the cause.
Titanium dioxide: A mineral sunscreen filter that protects mainly against UVB and short-wave UVA rays. It has a low risk of irritation; combined with zinc oxide, the protection becomes broader.
Tranexamic acid: An active ingredient used for melasma and stubborn dark spots, thought to reduce some of the signals that stimulate pigment production. On the skin it is used as a serum; taking it by mouth is only done on a doctor's decision.
Transepidermal water loss (TEWL): The amount of water lost by evaporating from inside the skin to its surface. It is one of the most important measures of barrier health; it rises as the barrier becomes damaged.
Tretinoin: Retinoic acid: the active form of vitamin A that acts directly on the skin. It is a prescription medication with strong evidence for acne and photoaging; it can cause irritation and sun sensitivity and is not used during pregnancy.

Urea: An ingredient that is part of the skin's natural moisturizing factor; it both holds moisture and, at higher concentrations, softens flakes. Concentrations of 5–10 percent are common in dry-skin care.
UVA: Long-wavelength ultraviolet rays from the sun. They pass through clouds and glass and reach the dermis; they are the main culprit behind collagen loss, pigmentation and photoaging.
UVB: Short-wavelength ultraviolet rays from the sun. They act in the epidermis; they are the main cause of sunburn and increase the risk of skin cancer. SPF mainly indicates UVB protection.

Vitamin C: A powerful antioxidant that plays a role in collagen production and protects against free radicals. It is also used for brightness and an even skin tone; its most effective but most delicate form is ascorbic acid, and gentler derivatives also exist.

Zinc oxide: A mineral sunscreen filter that reflects and absorbs UVA and UVB rays across a broad spectrum. Because its risk of irritation is low, it is often preferred for sensitive skin and for children.