Skincare Glossary
AHA (Alpha-Hydroxy Acid)
Water-soluble acids derived from fruits and milk that exfoliate the skin surface by dissolving bonds between dead cells. Glycolic acid (from sugarcane) and lactic acid (from milk) are the most common. Effective for texture, fine lines, and hyperpigmentation. Increase sun sensitivity.
BHA (Beta-Hydroxy Acid)
Oil-soluble acids that penetrate into pores to dissolve sebum and dead skin. Salicylic acid is the only commonly used BHA. Best for acne, blackheads, and oily skin. Also has anti-inflammatory properties.
Comedogenic
Tendency of an ingredient to clog pores and cause comedones (blackheads and whiteheads). Rated on a scale of 0 (non-comedogenic) to 5 (highly comedogenic). Individual reactions vary — comedogenic ratings are guides, not guarantees.
Emollient
A substance that softens and smooths the skin by filling gaps between dead skin cells. Emollients include oils, esters, and fatty acids (e.g., squalane, jojoba oil, isopropyl myristate). They improve skin texture and feel without necessarily adding moisture.
Humectant
A substance that attracts and holds water from the atmosphere and deeper skin layers into the stratum corneum. Common humectants include hyaluronic acid, glycerin, urea, and propylene glycol. Most effective when sealed with an occlusive.
Melanin
The pigment produced by melanocytes that gives skin its color and protects against UV damage. Overproduction causes hyperpigmentation. Eumelanin (brown/black) and pheomelanin (red/yellow) are the two main types. Tyrosinase is the key enzyme in melanin synthesis.
Occlusive
An ingredient that forms a physical barrier on the skin surface to prevent water evaporation (TEWL). Petrolatum is the most effective occlusive (reduces TEWL by 99%). Others include dimethicone, lanolin, mineral oil, and beeswax.
pH
A measure of acidity/alkalinity on a scale of 0-14. Healthy skin has a pH of 4.5-5.5 (the acid mantle). AHA/BHA products need pH 3-4 to be effective. Cleansers should be pH 4.5-6.5 to avoid disrupting the acid mantle. High-pH cleansers damage the skin barrier.
Retinization
The adjustment period when starting a retinoid, characterized by dryness, peeling, redness, and sensitivity. Typically lasts 4-12 weeks. Not damage — it is the skin adapting to increased cell turnover. Can be managed by starting slow, buffering with moisturizer, and using low concentrations.
Retinoid
Any derivative of vitamin A used in skincare. The retinoid family includes tretinoin (prescription, strongest), adapalene (OTC, moderate), retinol (OTC, mild), retinaldehyde (OTC, moderate), and retinyl palmitate (OTC, weakest). All ultimately convert to retinoic acid to exert effects.
Stratum Corneum
The outermost layer of the epidermis, composed of dead skin cells (corneocytes) held together by lipids (ceramides, cholesterol, fatty acids). This is the skin barrier. Most topical skincare ingredients must penetrate or interact with this layer to work.
TEWL (Transepidermal Water Loss)
The passive evaporation of water from inside the body through the skin to the external atmosphere. High TEWL indicates a damaged skin barrier. Measured in grams of water per square meter per hour. Moisturizers reduce TEWL through humectant, emollient, and occlusive mechanisms.
Tyrosinase
The rate-limiting enzyme in melanin synthesis. It catalyzes the conversion of tyrosine to DOPA and DOPA to dopaquinone — the first steps in melanin production. Most brightening ingredients (vitamin C, arbutin, kojic acid, azelaic acid) work by inhibiting tyrosinase.
UV Index
A measure of UV radiation intensity on a scale from 0 to 11+. At UV index 3 or above, sun protection is recommended. At 8+, sun exposure should be minimized. Sunscreen is recommended daily in areas that regularly exceed UV index 3.