Humectant

Components with the function of a humidifier help attract and retain water in the upper layers of the skin and on the surface of the hair. Due to this, they reduce the feeling of tightness, make the skin softer and visually smoother, and can give hair more elasticity, softness and a less dry appearance. In cosmetic formulas, such substances are important not only for care, but also for the consumer properties of the product: they maintain a comfortable distribution, reduce the risk of drying out after cleansing, and can affect the feeling of freshness and "watery" texture. The European CosIng database uses this function to classify cosmetic ingredients, and the FDA also classifies skin moisturizers as cosmetic products. (single-market-economy.ec.europa.eu)

The most typical examples are glycerin and urea. Glycerin is widely used in creams, serums, shampoos, and masks because it binds water well and helps the skin stay hydrated for longer. Urea is also known for its ability to retain moisture; in addition, depending on the concentration, it can additionally soften areas of dryness and roughness. In the same group, sodium hyaluronate, propylene glycol, sodium lactate and sorbitol are often found. The effect of such ingredients is especially noticeable in products for dry and dehydrated skin, as well as in the care of damaged or porous hair. (fda.gov)

In this case, the result depends on the entire formula, not just on one component. These substances work best in combination with emollient and protective ingredients that help reduce water loss from the skin's surface. Therefore, the moisturizer in the composition is not just a "water supplement", but an important part of the formula that affects both the condition of the skin and hair, and the comfort of using the product itself. (fda.gov)