A Callus

1. A hardened area of the skin (especially on the foot or hand) caused by repeated friction, wear or use. Callus - Wikipedia
2. The material of repair in fractures of bone; a substance exuded at the site of fracture, which is at first soft or cartilaginous in consistency, but is ultimately converted into true bone and unites the fragments into a single piece.
3. (botany) The new formation over the end of a cutting, before it puts out rootlets.
4. (entomology) A shining area on the frons of many species of Tabanomorpha (horse flies and relatives).