CALLUS

noun

1. A hardened area of the skin (especially on the foot or hand) caused by repeated friction, wear or use.
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. The new formation over the end of a cutting, before it puts out rootlets.

verb

1. To form such hardened tissue.