Hair is the filamentous outgrowth of the epidermis found in mammals. Hair is a characteristic of all mammals, though in some species hair is absent at certain stages of life. "Hairs" are also found on plants, the technical term for which is trichomes.

Hair serves a number of different functions. It provides insulation from cold weather and, in some species, from particularly hot weather. Because hair is often pigmented, it provides coloration. This might serve to camouflage an individual; in some mammals, the pigmentation changes with the seasons, becoming white during the snowy winter, for example.

In modern Western societies, it is considered manly for men to have hair on their faces, arms, chests and legs, but the hair growing from the top of the head is generally kept short, relatively speaking; equally, it is considered womanly for women to have no hair on their bodies, with the exception of pubic hair, but to have a lot of it on the tops of their heads. This is a fairly recent development. Before the First World War men generally had long hair and beards. The trench warfare between 1914 and 1918 exposed men to lice and flea infestation which caused the order to be given for the routine cutting of hair to a severely short length. The shorter style became the new normality and has never entirely gone away since.

The hair of non-human animal species is commonly referred to as fur.