In botany, soft tissue that forms over a wounded or cut plant surface, leading to healing. A callus arises from cells of the cambium. When a callus forms, some of its cells may organize into growing points, some of which in turn give rise to roots while others produce stems and leaves. Thus a callus may be capable of regenerating an entire plant.

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.

in osteology, bony and cartilaginous material forming a connecting bridge across a bone fracture during repair. Within one to two weeks after injury, a provisional callus forms, enveloping the fracture site. Osteoblasts (osteoblast), bone-forming cells in the periosteum (the bone layer where new bone is produced), proliferate rapidly, forming collars around the ends of the fracture, which grow toward each other to unite the fragments. The definitive callus forms slowly as the cartilage is resorbed and replaced by bone tissue. Two to three weeks after injury, strong bony extensions join the fractured bone ends, and the organized aspect of bone gradually recurs. The callus is resorbed over a period of months to years.