n (countable) A pigmented filament of keratin which grows from a follicle on the skin of humans and other mammals.
n (uncountable) The collection or mass of such growths growing from the skin of humans and animals, and forming a covering for a part of the head or for any part or the whole body.
n (zoology, countable) A slender outgrowth from the chitinous cuticle of insects, spiders, crustaceans, and other invertebrates. Such hairs are totally unlike those of vertebrates in structure, composition, and mode of growth.
n (botany, countable) A cellular outgrowth of the epidermis, consisting of one or of several cells, whether pointed, hooked, knobbed, or stellated.
n (countable) Any slender, flexible outgrowth, filament, or fiber growing or projecting from the surface of an object or organism.
n (countable, engineering, firearms) A locking spring or other safety device in the lock of a rifle, etc., capable of being released by a slight pressure on a hair-trigger.
n (obsolete) Haircloth; a hair shirt.
n (countable) Any very small distance, or degree; a hairbreadth.
n (slang, uncountable) Complexity; difficulty; the quality of being hairy.
n A surname.