glossary

Acid Mantle The thin, slightly acidic film on the surface of your skin (pH 4.5–5.5). It’s your skin’s first line of defence against bacteria, pollution, and irritants. Harsh cleansers and over-exfoliation are the most common things that disrupt it.
AHA (Alpha Hydroxy Acid) A water-soluble exfoliating acid that works on the surface of your skin, dissolving the bonds that hold dead skin cells together. Glycolic and lactic acid are the most common. Best for dull, uneven texture and dry skin types.
Antioxidant An ingredient that neutralises free radicals — unstable molecules caused by UV exposure, pollution, and stress that break down collagen and damage skin cells. Vitamin C, niacinamide, and vitamin E are the most well-studied.
Barrier Function Your skin’s ability to keep moisture in and irritants out. When barrier function is strong, skin feels comfortable, calm, and hydrated. When it’s compromised, everything else goes wrong.
BHA (Beta Hydroxy Acid) An oil-soluble exfoliating acid that can penetrate into pores, not just the surface. Salicylic acid is the main one. Particularly effective for congestion, blackheads, and oily or acne-prone skin.
Broad-Spectrum SPF Sunscreen that protects against both UVA rays (ageing, skin damage) and UVB rays (burning). If it doesn’t say broad-spectrum, it only covers one. You want both, every day.
Ceramides Lipids that make up roughly 50% of the “mortar” between your skin cells. They hold the barrier together and retain moisture. When ceramide levels drop — through age, damage, or harsh products — the barrier weakens. Replacing them topically is one of the most effective things you can do for compromised skin.
Cholesterol (in skincare) One of the three essential lipids in your skin barrier, alongside ceramides and fatty acids. It regulates how permeable the barrier is — how much gets in or out. Most moisturisers don’t include it, which is why products with all three lipids outperform ones with just ceramides.
Cleansing The act of removing makeup, SPF, pollution, and excess sebum from the skin. A good cleanser does this without stripping the acid mantle or disrupting the barrier. Your skin should feel comfortable — not tight — immediately after.
Closed Comedone A blocked pore that hasn’t opened to the surface — more commonly called a whitehead or a flesh-coloured bump under the skin. Caused by sebum and dead skin cells trapped in the follicle.
Collagen The structural protein that gives skin its firmness and elasticity. Production naturally slows from your mid-20s. UV exposure accelerates the breakdown. No topical product can meaningfully replace lost collagen — but protecting what you have (SPF, antioxidants) is genuinely effective.
Comedogenic A term used to describe ingredients thought to clog pores. Worth knowing, but the comedogenicity scales you’ll find online are largely based on outdated rabbit-ear studies and don’t reliably predict how a product will behave on human skin.
Contact Dermatitis Skin inflammation triggered by direct contact with a substance — either an irritant (like a harsh cleanser or over-exfoliated barrier) or an allergen (like a fragrance or preservative). Red, itchy, and sometimes weepy. Patch testing new products helps identify the cause.
Corneocytes The flattened, dead skin cells that make up the outermost layer of your skin (the stratum corneum). They’re not inert — they play an active role in barrier function and signalling. When the barrier is healthy, they lie flat and ordered. When it’s damaged, they lift and become irregular.
Desquamation The technical term for your skin’s natural shedding process — the cycle by which old skin cells rise to the surface and are shed. It happens continuously and invisibly when the barrier is healthy. Exfoliants accelerate it.
Double Cleansing A two-step cleansing method: an oil-based cleanser first (to remove SPF and makeup), followed by a water-based cleanser (to remove the rest). Useful if you wear SPF or makeup daily; unnecessary if you don’t.
Emollient An ingredient that softens and smooths skin by filling the gaps between skin cells. Fatty acids and squalane are good examples. Emollients improve texture and support the barrier but don’t actively draw water into the skin the way humectants do.
Epidermis The outermost of the three layers of your skin. It contains the stratum corneum (the barrier layer), melanocytes (which produce pigment), and Langerhans cells (which are part of the immune response). Most topical skincare works within this layer.
Exfoliation The process of removing dead skin cells from the surface, either with acids (chemical exfoliation) or abrasive particles (physical exfoliation). The right amount improves texture and brightness. Too much damages the barrier.
Fatty Acids Lipids that help fill the spaces between skin cells in the barrier — think of them as part of the mortar. Linoleic acid and oleic acid are the main ones in skin. Also found in oils: rosehip is high in linoleic acid; olive oil is high in oleic. The ratio matters for different skin types.
Filaggrin A protein that binds skin cells together and breaks down into natural moisturising factors (NMFs) — the compounds that keep the stratum corneum hydrated. People with eczema often have a genetic mutation that reduces filaggrin production, which is a key reason their barrier is inherently more vulnerable.
Fragrance (in skincare) A common allergen and one of the most frequent causes of contact dermatitis and barrier disruption. Both synthetic fragrance and “natural” fragrance (including essential oils) can trigger reactions. Fragrance-free is the safest choice for sensitive or compromised skin.
Free Radicals Unstable molecules generated by UV exposure, pollution, smoking, and other environmental stressors. They attack skin cells, break down collagen, and accelerate ageing. Antioxidant ingredients neutralise them before they can cause damage.
Glycerin One of the most effective and well-studied humectants in skincare. Draws water from the air and deeper layers of skin into the stratum corneum. Cheap, non-irritating, and widely available. Found in most decent moisturisers.
Humectant An ingredient that attracts and holds water — either from the air or from deeper skin layers. Hyaluronic acid, glycerin, and urea are the main ones. Humectants hydrate but don’t seal: you need an occlusive on top to lock the moisture in.
Hyaluronic Acid (HA) A naturally occurring humectant that can hold up to 1,000 times its weight in water. Often overhyped. It works best in humid environments and when layered under a moisturiser — applied to dry skin in a dry climate, it can pull moisture out of the skin instead of into it.
Hyperpigmentation Patches of skin that are darker than the surrounding area, caused by excess melanin production. Triggers include sun exposure, post-inflammatory response (after acne or injury), and hormonal changes. SPF is the most important treatment and the most important prevention.
Inflammation The skin’s immune response to perceived damage or threat — redness, warmth, swelling. Acute inflammation is protective. Chronic, low-grade inflammation (from a disrupted barrier, over-exfoliation, or UV exposure) contributes to premature skin ageing and sensitivity.
Keratinocyte The most common cell type in the epidermis. Keratinocytes form at the base of the skin, gradually rise to the surface, and eventually flatten into the dead corneocytes that make up the stratum corneum. The journey takes roughly 28 days in younger skin, longer as we age.
Lactic Acid An AHA that exfoliates the surface of the skin. Gentler than glycolic acid, with some humectant properties. A good entry point for beginners to chemical exfoliation, and particularly useful for dry or sensitive skin types.
Lipid Barrier The layer of fats (lipids) — ceramides, cholesterol, and fatty acids — that fills the spaces between skin cells in the stratum corneum. It’s the barrier’s waterproofing system. Disrupt the lipid barrier and water escapes, irritants get in, and the skin becomes reactive.
Melanin The pigment produced by melanocytes in the skin that gives it its colour and absorbs UV radiation. More melanin means more natural UV protection, but all skin tones need SPF — darker skin is not immune to UV damage or skin cancer.
Microbiome (Skin) The community of bacteria, fungi, and other microorganisms that live on the surface of your skin. A balanced microbiome supports the barrier and immune function. Harsh cleansing, over-exfoliation, and certain ingredients can disrupt it.
Moisturiser A product that supports the skin’s hydration in one or more of three ways: humectants attract water, emollients soften and smooth, and occlusives seal moisture in. The best barrier-repair moisturisers do all three.
Natural Moisturising Factors (NMFs) A mix of water-attracting compounds — amino acids, urea, lactic acid, and others — found naturally in the stratum corneum. They keep the outer skin layer pliable and hydrated. They’re depleted by harsh cleansers and over-exfoliation.
Niacinamide A form of vitamin B3 with a strong evidence base. Improves barrier function, reduces the appearance of pores and uneven tone, calms redness, and regulates sebum. One of the most forgiving and versatile actives in skincare — almost no irritation risk.
Non-Comedogenic A label claiming a product won’t clog pores. It’s not regulated, standardised, or reliably tested on human skin. Worth noting, but not worth relying on exclusively.
Occlusives Ingredients that sit on top of the skin and form a physical seal to reduce water loss. Petroleum jelly is the most effective. Beeswax, shea butter, and dimethicone also function as occlusives. Essential for anyone with a compromised barrier.
Open Comedone A blocked pore exposed to air — the sebum inside oxidises and turns dark, which is why they’re called blackheads. Not caused by dirt. Salicylic acid is the most effective ingredient for managing them.
Over-Exfoliation Using exfoliating acids or scrubs too frequently, or at too high a concentration, stripping the barrier faster than it can repair itself. One of the most common causes of reactive, sensitised skin. Signs include stinging, redness, and products that suddenly feel uncomfortable on skin they previously worked fine on.
Oxidative Stress Damage to skin cells caused by free radicals — the result of UV exposure, pollution, smoking, and other environmental factors. A key driver of premature skin ageing. Antioxidants are the main defensive tool.
Patch Testing Applying a small amount of a new product to a discreet area of skin — typically the inner arm or behind the ear — and waiting 24–48 hours before applying it to the face. Reduces the risk of a full-face reaction to a new ingredient.
Peptides Short chains of amino acids that act as signalling molecules in the skin. Some have good evidence for supporting collagen synthesis or barrier repair. Quality varies enormously between products; the evidence base is more limited than marketing often suggests.
pH A scale from 0–14 measuring acidity or alkalinity. Your skin’s surface is naturally slightly acidic (pH 4.5–5.5). Products with a pH too far above this disrupt the acid mantle. AHAs and BHAs only work effectively at a low pH (around 3.5–4). Cleansers that feel “squeaky clean” are usually too alkaline.
Physical Exfoliation Using a physical abrasive — a scrub, brush, or cloth — to manually remove dead skin cells. Less precise than chemical exfoliation and more likely to cause micro-tears in the skin if the abrasive is coarse. Generally not recommended for sensitive or reactive skin.
Post-Inflammatory Hyperpigmentation (PIH) Darkening of the skin following inflammation — most commonly after acne, a scratch, or any wound. The skin produces excess melanin as part of the healing response. More common in darker skin tones. SPF is the most important factor in preventing it from deepening.
Purging A temporary increase in breakouts that can occur when starting a new active — typically a retinoid or exfoliating acid — that accelerates cell turnover. True purging happens in areas where you already break out and clears within 4–6 weeks. Breakouts in new areas, or those that persist beyond 6 weeks, are reactions, not purging.
Retinol A vitamin A derivative and one of the most evidence-backed actives in skincare. Increases cell turnover, stimulates collagen production, and improves texture and tone over time. Requires a slow introduction — too much too soon causes barrier disruption, redness, and peeling.
Retinoids The umbrella term for all vitamin A derivatives, ranging from prescription-strength tretinoin to over-the-counter retinol and retinal. More potent forms work faster but carry a higher irritation risk. All require an adjustment period and diligent SPF use.
Rosacea A chronic inflammatory skin condition that causes persistent redness, visible blood vessels, and sometimes acne-like breakouts, primarily on the cheeks, nose, chin, and forehead. Different from a compromised barrier, though the two can look similar. Requires a dermatologist’s diagnosis and specific management.
Salicylic Acid A BHA that exfoliates inside the pore as well as on the surface. Oil-soluble, so it penetrates sebum effectively. The go-to for blackheads, congestion, and acne-prone skin. Not ideal for dry or sensitive skin types.
Sebum The natural oil produced by sebaceous glands in the skin. At the right levels, it moisturises and protects the skin. Too much leads to congestion; too little (often caused by harsh cleansers) leads to dryness and barrier disruption.
Sensitised Skin Skin that has become reactive due to external damage — over-exfoliation, harsh products, environmental stress — rather than an inherent genetic trait. Unlike sensitive skin (a skin type), sensitisation is reversible. Stripping back to the basics and rebuilding the barrier usually resolves it.
Squalane A stable, lightweight emollient derived from sugarcane or olives (the older, less sustainable source was shark liver). Closely mimics the skin’s natural lipids, making it well-tolerated by almost all skin types. Softens, smooths, and helps maintain barrier integrity without feeling greasy.
Stratum Corneum The outermost layer of the epidermis — the part of your skin you can see and touch. Made up of dead, flattened skin cells (corneocytes) embedded in a lipid matrix. This is the skin barrier. Everything in skincare either supports it, works within it, or damages it.
Sunscreen (Chemical) Works by absorbing UV radiation and converting it to heat. Tends to be lightweight and invisible on skin. Some chemical filters (like avobenzone) can irritate sensitive skin. Common filters: oxybenzone, octinoxate, tinosorb.
Sunscreen (Mineral) Works by sitting on the surface of the skin and physically reflecting UV rays. The two mineral filters are zinc oxide and titanium dioxide. Less likely to irritate — often recommended for sensitive or rosacea-prone skin. Newer formulations have significantly reduced the white cast.
Toner A liquid applied after cleansing, originally designed to restore pH after alkaline soaps disrupted the acid mantle. Most modern cleansers are pH-balanced, making traditional toning unnecessary. Some modern toners deliver humectants or exfoliating acids and can be useful — it depends on what’s in them, not the format itself.
Transepidermal Water Loss (TEWL) The process by which water evaporates through the skin into the air. Some TEWL is normal. Elevated TEWL is a sign of a compromised barrier — the seal is broken and the skin can’t hold onto moisture. Occlusives reduce TEWL by physically blocking evaporation.
Tretinoin Prescription-strength retinoic acid — the most potent and well-studied form of vitamin A available topically. The evidence for it improving texture, tone, pigmentation, and collagen production is stronger than for any OTC retinoid. Requires a prescription in most countries and a careful introduction period.
Urea A naturally occurring compound in the skin’s Natural Moisturising Factors that draws water into the skin (humectant) and gently loosens dead skin cells at higher concentrations (keratolytic). Often overlooked but highly effective, particularly for dry or rough skin. Well-tolerated and inexpensive.
Vitamin C A potent antioxidant that neutralises free radicals and plays a key role in collagen synthesis. L-ascorbic acid is the most studied form but is unstable and can oxidise quickly. Many derivative forms (ascorbyl glucoside, magnesium ascorbyl phosphate) are more stable but have a weaker evidence base. Best applied in the morning, before SPF.
Vitamin E (Tocopherol) An antioxidant that works well alongside vitamin C — the combination is more effective than either alone. Also has emollient and anti-inflammatory properties. Found naturally in the skin’s sebum. A common addition to moisturisers and facial oils.
Zinc Oxide A mineral UV filter that reflects both UVA and UVB rays. One of the safest and best-tolerated sunscreen ingredients — frequently recommended for sensitive and reactive skin. Also has mild anti-inflammatory properties, which is why it appears in products for acne-prone skin.