1. (in botany) A tissue consisting of undifferentiated totipotent cells that develops over a cut or damaged plant surface. Hence, callus forms at the cut end of a cutting and can give rise to adventitious roots, either spontaneously or following treatment with hormone ‘rooting’ powder—a key technique of vegetative propagation. The dedifferentiation of tissue cells to form undifferentiated callus is used as the basis for the commercial cloning of many plant species. By culturing the callus in specialized media containing appropriate nutrients and plant hormones, new genetically identical embryos are formed, which develop into new plants.

2. (in pathology) A thick hard area of skin that commonly forms on the palms of the hands and soles of the feet as a result of continuous pressure or friction.

3. (in physiology) Hard tissue formed round bone ends following a fracture, which is gradually converted to new bone.