1. Callus is connective tissue initially formed around a fracture (see fractures and dislocations), and slowly ossified as repair proceeds. See also bone.
2. In botany, callus is a protective mass of undifferentiated cells formed at the site of a wound in a woody plant. Callus tissue is also formed at the base of cuttings as they start to take root. Callus tissue is important as the starting point for tissue culture of plants.