Skincare terms, explained.

AHAs (alpha hydroxy acids) and BHAs (beta hydroxy acids) are exfoliating acids that loosen the top layer of dead skin cells and smooth the surface.

Broad-spectrum sunscreen protects skin against both UVB and UVA rays, the two types of UV light that reach the ground.

Ceramides are waxy lipid molecules that make up a large share of the skin barrier, helping skin cells stick together and keeping water in.

Clean beauty is a marketing term with no legal definition. Brands use it to mean formulas without certain ingredients, but each company sets its own standard.

In skincare, a peptide is a short chain of amino acids that is claimed to signal skin cells, for example to support collagen production.

Cruelty-free means a company does not test its finished products or ingredients on animals. Third-party programmes such as Leaping Bunny check the claim.

An emollient softens skin by filling the gaps between skin cells with lipids (fats), which smooths the surface and makes it more flexible.

Fragrance-free means a product has no added fragrance; unscented means a fragrance has been added to cover the smell of other ingredients.

Glass skin is a Korean beauty term for skin that looks very smooth, hydrated and luminous, like polished glass. It describes a look, not a medical condition.

Glutathione is a natural antioxidant that the body makes from three amino acids, and it is used in skincare to counter oxidative stress.

Glycation is a reaction in which sugar molecules bond to proteins such as collagen, forming advanced glycation end-products (AGEs) that make tissue stiffer.

Glycerin is a simple, colourless humectant that draws water into the outer skin and is one of the most widely used moisturising ingredients.

A humectant is an ingredient that attracts water and holds it in the outer skin.

Hyaluronic acid is a sugar molecule found naturally in skin that binds many times its own weight in water, and it is widely used in skincare as a humectant.

Hydrolyzed collagen is collagen broken down into smaller peptide fragments, which are used in skincare to help skin hold moisture.

A marine bio-active is a compound from seaweed, algae or other marine organisms, used in skincare for its biological effect on skin.

Niacinamide is the amide form of vitamin B3, used in skincare to support the skin barrier and calm redness.

Non-comedogenic means a product is labelled as unlikely to clog pores, but there is no standard test or legal definition behind the term.

An occlusive forms a film on the skin that slows water loss from it.

Photoaging is skin aging caused by repeated exposure to ultraviolet (UV) light, which causes wrinkles, dark spots, roughness and loss of elasticity.

Retinoids are vitamin A derivatives, such as retinol and prescription tretinoin, that speed up skin cell turnover and are among the best-studied anti-aging topicals.

The skin barrier is the outermost layer of the epidermis, the stratum corneum, which keeps water in and irritants out.

Skin pH is the acidity of the skin’s surface, usually about 4.5 to 5.5, and the acid mantle is the thin acidic film that helps protect the barrier.

SPF, or sun protection factor, is the number on a sunscreen label that rates how well it protects against UVB rays, the main cause of sunburn.

A topical antioxidant is an ingredient applied to the skin that neutralizes reactive oxygen molecules, which build up from sun exposure and pollution.

Transepidermal water loss (TEWL) is the amount of water that passes through the skin and evaporates into the air.

Topical vitamin C, usually as ascorbic acid or a stable derivative, is an antioxidant used in serums to support collagen and protect against UV-related oxidative damage.