1. (noun)hair a covering for the body (or parts of it) consisting of a dense growth of threadlike structures (as on the human head); helps to prevent heatloss "he combed his hair"; "each hair consists of layers of dead keratinized cells"
2. (noun)hair's-breadth, hairsbreadth, hair, whisker a very smalldistance or space "they escaped by a hair's-breadth"; "they lost the election by a whisker"
3. (noun)hair, fuzz, tomentum filamentous hairlike growth on a plant "peach fuzz"
4. (noun)hair, pilus any of the cylindrical filaments characteristically growing from the epidermis of a mammal "there is a hair in my soup"
Definition of 'hair' 
