in mammals, the characteristic threadlike outgrowths of the outer layer of the skin (epidermis) that form an animal's coat, or pelage. Hair is present in differing degrees on all mammals. On adult whales, elephants, sirenians, and rhinoceroses body hair is limited to scattered bristles. In most other mammals the hair is abundant enough to form a thick coat, while humans are among the most hairless of all mammals.

The most important function of hair in mammals is that of insulating against cold by conserving body heat. The differing colours and colour patterns in hair coats can also serve purposes of camouflage and of sexual recognition and attraction among the members of a species. Specialized hairs called vibrissae, or whiskers, serve as sensory organs for certain nocturnal animals. The specially modified hairs of the porcupine are called quills and serve defensive purposes.

Human beings have several different types of hairs. The first to develop is the lanugo, a layer of downy, slender hairs that begin growing in the third or fourth month of fetal life and are entirely shed either before or shortly after birth. During the first few months of infancy there grow fine, short, unpigmented hairs called down hair, or vellus. Vellus covers every part of the body except the palms of the hands, the soles of the feet, undersurfaces of the fingers and toes, and a few other places. At and following puberty, this hair is supplemented by longer, coarser, more heavily pigmented hair called terminal hair that develops in the armpits, genital regions, and, in males, on the face and sometimes on parts of the trunk and limbs. The hair of the scalp, eyebrows, and eyelashes are of separate types from these others and develop fairly early in life. On the scalp, where hair is usually densest and longest, the average total number of hairs is between 100,000 and 150,000. Human hair grows at a rate of about 0.5 inch (13 mm) per month.

The typical mammalian hair consists of the shaft, protruding above the skin, and the root, which is sunk in a pit (follicle) beneath the skin surface. Except for a few growing cells at the base of the root, the hair is dead tissue, composed of keratin and related proteins. The hair follicle is a tubelike pocket of the epidermis that encloses a small section of the dermis at its base. The human hair is formed by divisions of cells at the base of the follicle. As the cells are pushed upward from the follicle's base, they become keratinized (hardened) and undergo pigmentation.

Hair is continually shed and renewed by the operation of alternating cycles of growth, rest, fallout, and renewed growth. The average life of different varieties of hair varies from about 4 months for downy hairs to 3 to 5 years for long scalp hairs. Each human follicle follows this cycle independently of others, so the total amount of hair remains constant; some animals' hair follicles have synchronous cycles, causing periodic shedding, or molts.