Skincare terms, explained simply

AHA (Alpha Hydroxy Acid)

A group of water-soluble acids (glycolic, lactic, mandelic) used to exfoliate the skin surface. They work by loosening the bonds between dead skin cells, promoting turnover and a smoother texture.

BHA (Beta Hydroxy Acid)

An oil-soluble acid (salicylic acid) that penetrates into pores to help clear congestion. Commonly used for acne-prone and oily skin types.

Barrier function

The skin's outermost layer (stratum corneum) acts as a barrier against moisture loss and environmental irritants. When compromised, skin can become dry, reactive, and more vulnerable to irritation.

Ceramides

Lipids naturally found in the skin barrier that help retain moisture and protect against environmental damage. Often included in moisturisers to support barrier repair.

Comedone

A clogged pore. Open comedones are blackheads; closed comedones are whiteheads. They form when sebum and dead skin cells block the hair follicle.

Dark circles

Darkness beneath the eyes caused by pigmentation, visible blood vessels, volume loss, or shadowing. One of the most sleep-responsive skin metrics.

TEWL (Transepidermal Water Loss)

The rate at which water evaporates from the skin surface. Higher TEWL indicates a weaker barrier. Stella tracks visible indicators of barrier health rather than TEWL directly.

Hyperpigmentation

Patches of skin that appear darker than the surrounding area due to excess melanin production. Can be triggered by UV exposure, inflammation, hormonal changes, or injury.

ITA (Individual Typology Angle)

A measurement of skin lightness used to classify skin tone objectively. Stella uses ITA as a baseline property to contextualise other pigmentation metrics.

Melasma

A form of hyperpigmentation that appears as brown or grey patches, typically on the cheeks, forehead, or upper lip. Often influenced by hormones, UV exposure, and heat.

Niacinamide

A form of vitamin B3 used in skincare. Associated with supporting barrier function, reducing the appearance of pores, and improving uneven skin tone.

PIE (Post-Inflammatory Erythema)

Red or pink marks left on the skin after a breakout or irritation heals. Caused by damaged blood vessels near the surface. Different from PIH, which is pigmented.

PIH (Post-Inflammatory Hyperpigmentation)

Dark marks left on the skin after inflammation or injury. More common in darker skin tones. Can take months to fade without intervention.

Retinoid

A class of vitamin A derivatives (retinol, retinal, tretinoin) used in skincare. Associated with supporting cell turnover, reducing the appearance of fine lines, and improving texture.

Sebum

An oily substance produced by sebaceous glands. Helps protect and moisturise the skin naturally. Excess production can contribute to oiliness and breakouts.

Skin barrier

See: Barrier function. The outermost protective layer of your skin that prevents moisture loss and shields against irritants, pollutants, and pathogens.

SPF (Sun Protection Factor)

A measure of how well a sunscreen protects against UVB radiation. SPF 30 blocks about 97% of UVB rays; SPF 50 blocks about 98%. Reapplication and coverage matter as much as the number.

Squalane

A lightweight, stable oil derived from squalene. Mimics the skin's natural lipids and is commonly used to support hydration and barrier function without clogging pores.

Telangiectasia

Visibly dilated blood vessels near the skin surface, often appearing as thin red lines. Common around the nose and cheeks. Can be more visible when the skin barrier is compromised.

Vitamin C (L-Ascorbic Acid)

An antioxidant commonly used in skincare serums. Associated with supporting brightness, reducing the appearance of pigmentation, and providing photoprotection alongside SPF.