Callus: 1. A localized firm thickening of the upper layer of skin as a result of repetitive friction. A callus on the skin of the foot has become thick and hard from rubbing (as a result of repetitive friction). Calluses of the feet may lead to other problems such as serious infections. Shoes that fit well can keep calluses from forming on the feet.
2. The hard new bone substance that forms in an area of bone fracture. Bony callus is part of the bone repair process.
Also called, less often, a callosity. Both callus and callosity are derived from the Latin callum meaning thick skin.