Hair is a filamentous outgrowth of the skin found only in mammals. In some species it is absent at certain stages of life. It projects from the epidermis, though it grows from follicles deep in the dermis. So-called "hairs" (trichomes) are also found on plants. The projections on insects and spiders are actually bristles. The hair of non-human species is commonly referred to as fur. There are varieties of cats, dogs, and mice bred to have little or no visible hair.

Hair serves a number of different functions. It provides insulation from the cold, and in some species from hot weather. It is generally pigmented, providing coloration, sometimes the same as the underlying skin. It often serves as camouflage, both for prey and predators. In some species the pigmentation changes with the seasons; e.g., becoming white during the snowy winter, and in cases even more rapidly than that with changes in background. Hair can also provide protection against abrasion, and head hair can buffer impacts to the skull. In some species, hair patterns can be a part of sexual dimorphism; e.g., the long manes of male lions.

In modern Western societies it is considered masculine for men to maintain the naturally thicker hair on their faces, arms, chests, backs, buttocks and legs, but the hair growing from the top of the head is generally kept relatively short. By contrast, it is considered feminine, for women to have little or no hair on their bodies, including pubic hair, but to let it grow long on the tops of their heads. Before World War I men generally had longer hair and beards. The trench warfare between 1914 and 1918 exposed men to lice and flea infestations, which caused the order for hair to be cut short, establishing a norm that has persisted. Hair care for humans is a major world industry with specialized tools, chemicals and techniques. In most societies, people style or adorn their hair for aesthetic reasons and often have it cut or removed by shaving or other means. In some, women usually shave their legs, armpits and the entirety or just parts of the pubic area, and shape their eyebrows.

Table of Contents:
Human hair
Types of hair
Hair change with aging
Androgenic hair