also spelled  Callous,  in dermatology, small area of thickened skin, caused by continued friction, pressure, or other physical or chemical irritants. In response to mild but repeated injury, the cells of the epidermis, the outermost horny layer of the skin, become more active, giving rise to a localized increase in horny tissue on the surface of the skin. Calluses are most frequently seen on the hands and feet. They are usually yellowish white, flat, and painless. When a callus is conical in shape, penetrating into the deeper layer of the skin and causing pain when pressed, it is called a corn.