Etymology

, , , from .
A cognate of the latter seems to be Lithuanian .
From the same Germanic word come, indirectly, Old Frisian , Dutch , German , Old High German , Icelandic and Swedish .

Noun

1. 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.

2. 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.

3. 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.
4. 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).
5. A haircloth. - Geoffrey Chaucer
6. Any very small distance, or degree; a hairbreadth.

   Just a little louder please - turn that knob a hair to the right.

Usage notes

The word hair is usually used without article in singular number when it refers to all the hairs on one's head in general. But if it refers to a single hair, it takes the indefinite article (a hair), or if it refers to more than one hair, a few hairs, then it takes the plural form without an article, and needs a plural verb.

George has (-) brown hair, but I found a hair on the sofa and suspect he's getting some gray hairs.

George's hair is brown, but one hair I found was grey, so I think there are probably more grey hairs on his head as well.