Glossary

Accord

A combination of materials in a fragrance that reads as one new smell rather than as its parts, the way a chord reads as more than its pitches. The fougere and amber accords are the classic examples. See fragrance notes explained.

AHA (alpha hydroxy acid)

A family of water-soluble exfoliating acids that loosen the bonds between dead surface cells. Glycolic acid is the smallest and most penetrating; lactic acid is larger and gentler.

Base note

The least volatile materials in a fragrance, which persist longest and define how it ends. Woods, resins, musks, vetiver, patchouli and oud live here.

BHA (beta hydroxy acid)

In cosmetics, effectively always salicylic acid. Oil-soluble, so it penetrates sebum and exfoliates inside the pore rather than only on the surface.

Chypre

A structural fragrance family built on a citrus top, a labdanum heart and an oakmoss base, rather than on a single dominant material. See chypre.

Comedogenic

Prone to causing comedones — the plugs of sebum and dead cells that become blackheads and closed bumps. Ratings come from crude assays and predict individual reactions poorly.

Emollient

An ingredient that fills the gaps between surface skin cells so skin feels smooth. Squalane is the archetype. Distinct from a humectant and from an occlusive.

Encapsulation

Wrapping an unstable or irritating active in a carrier that releases it slowly. Common with retinol and retinal; it usually improves tolerance at a given percentage, and it makes label percentages non-comparable across products.

Fougere

A structural fragrance family built on lavender, coumarin and oakmoss. The backbone of classical masculine perfumery. See fougere.

Free acid value

How much of an exfoliating acid is unbuffered and chemically active at the product's pH. It determines potency far more than the printed percentage, and almost nobody publishes it.

Humectant

An ingredient that binds water and holds it at the skin surface. Hyaluronic acid, glycerin and polyglutamic acid. Needs an occlusive over it or it can pull water out of deeper skin in dry air.

Keratolytic

Loosening or dissolving the keratin bonds that hold dead surface cells together. Exfoliating acids are keratolytic; so, mildly, is azelaic acid.

Melasma

A pattern of symmetrical facial pigmentation driven partly by hormonal and vascular signaling rather than by UV alone. Responds better to tranexamic acid than to simple tyrosinase inhibitors, and relapses without sun protection.

Occlusive

An ingredient that forms a physical barrier slowing water loss from the skin surface. Petrolatum is the strongest; ceramides and squalane act more lightly.

Photostability

Whether an ingredient survives exposure to light. Retinol and L-ascorbic acid are not photostable, which is why one is a night product and the other needs opaque packaging.

PIH (post-inflammatory hyperpigmentation)

The flat brown marks left behind after a spot or other inflammation heals. Distinct from scarring, which is structural. Responds to azelaic acid, niacinamide and vitamin C.

Projection

How far a fragrance travels from the wearer at a given moment. Distinct from sillage, which is the trail left behind. See sillage vs. projection.

Purging

A temporary increase in breakouts when an active accelerates turnover and brings existing microcomedones to the surface. Typically two to six weeks. Sustained new breakouts after that are a reaction, not a purge.

Retinization

The adjustment period at the start of retinoid use — flaking, tightness, mild stinging — as turnover accelerates. Expected and temporary. Sustained burning or a spreading rash is not retinization.

Retinoid

The family of vitamin A derivatives, not a single ingredient. In order of proximity to the active form: retinyl esters, retinol, retinal, then retinoic acid (tretinoin). See retinol vs. tretinoin.

Sillage

The scented trail a fragrance leaves behind as the wearer moves. From the French for a boat's wake. See sillage vs. projection.

Squalane vs squalene

One letter, two molecules. Squalene is the unstable lipid your own sebum contains; squalane is the hydrogenated, shelf-stable cosmetic version. See squalane vs. squalene.

Stratum corneum

The outermost layer of skin: dead corneocytes held in a lipid matrix. The bricks-and-mortar model. Damage to the mortar is what a compromised barrier means.

Top note

The most volatile materials in a fragrance, perceived in the first fifteen to thirty minutes. Citrus and light aromatics. The reason a first spray never represents the fragrance.

Tyrosinase

The enzyme that catalyses the first step of melanin production. Inhibiting it is the mechanism behind most brightening ingredients, including vitamin C and, selectively, azelaic acid.

Vehicle

What an active is carried in — serum, cream, gel, oil, suspension. It frequently matters more than the last few percent of concentration, and it is rarely discussed on a label.

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