callus

Short definition from Wiktionary

Noun

1. 1
   A hardened area of the skin (especially on the foot or hand) caused by repeated friction, wear or use.
2. 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. 3
   The new formation over the end of a cutting, before it puts out rootlets.
4. 4
   In orchids, a fleshy outgrowth from the labellum.
5. 5
   In grasses, a hardened extension from the base of a floret, which may or may not elongate and is often covered in hairs or bristles.
6. 6
   A shining area on the frons of many species of Tabanomorpha (horse flies and relatives).

Verb

1. 1
   To form such hardened tissue.