: a slender threadlike outgrowth of the epidermis of an animal; especially: one of the usually pigmented filaments that form the characteristic coat of a mammal
: the hairy covering of an animal or a body part; especially: the coating of hairs on a human head
Illustration of HAIR
Threadlike outgrowths of the skin. Babies shed a layer of downy, slender hairs (lanugo) before or just after birth. The fine, short, unpigmented hairs (vellus) then grow. Starting at puberty, terminal hair, longer, coarser, and more pigmented, develops in the armpits, crotch, sometimes on parts of the trunk and limbs, and, in males, on the face. Scalp hair, eyebrows, and eyelashes are different types. The number of scalp hairs, which grow about 0.5 in. (13 mm) per month, averages 100,000–150,000. The hair shaft (above the skin) is dead tissue, composed of keratin. Only a few growing cells at the base of the root are alive. Hair is formed by cell division at the base of the follicle (a tiny pocket in the skin), part of a cycle of growing, resting, and falling out. Vellus lasts about four months, scalp hairs three to five years.