(countable, uncountable) A pigmented keratinaceous growth that forms thin spires and grows out from a follicle on the human head, or the collection of them.
Then read he me how Sampson lost his hairs. - Geoffrey Chaucer
And draweth new delights with hoary hairs. - Edmund Spenser
She said she couldn't go out with me Friday - she had to wash her hair.
(uncountable) The collection or mass of filaments 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.
The Hair on a bear makes a warm fur coat.
(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.
(botany, countable) A cellular outgrowth of the epidermis, consisting of one or of several cells, whether pointed, hooked, knobbed, or stellated. Internal hairs occur in the flower stalk of the yellow frog lily (Nuphar).
(obsolete) A haircloth. - Geoffrey Chaucer
(countable) Any very small distance, or degree; a hairbreadth.
Just a little louder please - turn that knob a Hair to the right.