Hair is an outgrowth of filamentous cells, containing keratin, that grows from follicles found in the dermis. The human body, apart from the palms of the hands and soles of the feet, is covered in follicles which produce thick terminal and fine vellus hair. Most common interest in hair is focused on hair growth, hair types and hair care but hair is also an important biomaterial and the hair follicle is a well studied biological organ.

Found exclusively in mammals, hair is one of the defining characteristics of the mammalian class. Although other non-mammals, especially insects, show filamentous outgrowths, these are not considered "hair" in the scientific sense. So-called "hairs" (trichomes) are also found on plants. The projections onarthropods such as insects and spiders are actually insect bristles, composed of a polysaccharide calledchitin. There are varieties of dogs, cats, and mice bred to have little or no visible fur. In some species, hair is absent at certain stages of life. The main component of hair fiber is keratin.

Hair often refers to two distinct structures (1) the part beneath the skin, called the hair follicle or when pulled from the head called the bulb. This organ is located in the dermis and maintains stem cells which not only re-grow the hair after it falls out but also are recruited to regrow skin after a wound; and (2) the shaft, which is considered the part of the hair above the skin surface though it appears first in the epidermis. The hair shaft, in cross-section, can be divided roughly into three zones. Starting from the outside, (1) the cuticle which consists of several layers of flat, thin cells laid out like roof shingles, (2) the cortex which contain the keratin bundles in cell structures that remain roughly rod like and in some cases (3) the medulla, a disorganized and open area at the fibers center.

Cross section of a hair