Hair: An accessory organ of the skin. A hair is composed of keratinized cells compactly cemented together. Hair is distributed over almost the entire body. It consists of the root, or portion below the surface, and the shaft, or portion extending above the surface. The root is embedded in a pit-like depression called the hair follicle. It widens at its lower end to include small vascular papillae (small loops of blood vessels). These papillae provide nutrition for the growth of hair. Hair grows as a result of the division of the cells of the root. The hair follicle is placed obliquely in the skin, and a small smooth muscle called the arrector muscle, fastened to the side, causes the hair to stand up in cold or fright and gives the appearance of goose flesh to the skin.