1. A multicellular threadlike structure, consisting of many dead keratinized cells, that is produced by the epidermis in mammalian skin. The section of a hair below the skin surface (the root) is contained within a hair follicle, the base of which produces the hair cells. Hair assists in maintaining body temperature by reducing heat loss from the skin. Bristles and whiskers are specialized types of hair.

2. Any of various threadlike structures on plants, such as a trichome.