Hair is a filamentous outgrowth of dead cells from the skin, found only in mammals. Although many other life forms, especially insects, show filamentous outgrowths, these are not considered "hair" regarding the accepted meaning of the term. The primary component of hair fiber is keratin. Keratins are proteins, long chains (polymers) of amino acids. Keratin proteins form the cytoskeleton (miniature skeleton within a cell) of all epidermal cells. Keratin filaments run within a cell from the inside of the outer membrane to weave a "basket" around the nucleus of the cell. Keratins are a principle part of the cells in the epidermis, hair, nails, feathers, hooves, horny tissues and the enamel of teeth.

The average hair has about 100,000 hair follicles.Its absence is termed alopecia, commonly known as baldness. Anthropologists speculate that the functional significance of long head hair may be adornment, a byproduct of secondary natural selection once other somatic hair had been lost.

Another possibility is that long head hair is a result of Fisherian runaway sexual selection, where long lustrous hair is a visible marker for a healthy individual.

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