1. 1. Any of the cylindrical, keratinized, often pigmented filaments characteristically growing from the epidermis of a mammal.
   2. A growth of such filaments, as that forming the coat of an animal or covering the scalp of a human.
2. A filamentous projection or bristle similar to a hair, such as a seta of an arthropod or an epidermal process of a plant.
3. Fabric made from the hair of certain animals: a coat of alpaca hair.
4. 1. A minute distance or narrow margin: won by a hair.
   2. A precise or exact degree: calibrated to a hair.

[Middle English her, from Old English hǣr.]

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.