Horn, a hard, flexible, translucent material that is formed from cells of the outer layer of skin, or epidermis. Projections of bone covered with horn, which grow on the heads of cattle, sheep, goats, bison, buffalo, and antelopes, are called horns. Deer antlers are sometimes called horns, but they are of solid bone without a horny covering. The horns on the nose of the rhinoceros are made up of solid masses of modified hair without a core of bone.

Fingernails, toenails, claws, and hoofs are made of horn. Other horny growths include the scales of reptiles; whalebone, from the mouths of baleen whales; and the horny covering of birds' beaks and the bony shells of most turtles.

The chemical basis of horn is keratin, a fibrous protein that accumulates in the epidermal cells. Birds' feathers and the scales on the shafts of human hair are composed of horny, or keratinized, tissue. If skin is exposed to constant pressure and friction, horny tissue may accumulate and produce corns.

Horn has been used since prehistoric times for making useful and ornamental objects. Stone Age artisans carved pictures on animal horns. For centuries horns were used as drinking cups, and as containers for food, medicines, and gunpowder. They were also used as musical instruments and for communicating outdoors. Horn from tortoise shells was made into “horn rims” for eyeglasses and into handles for umbrellas and walking sticks. Buttons, combs, and knife handles were also made of horn. Synthetic plastic materials have replaced horn in the manufacture of many of these objects.