The Collaborative International Dictionary of English v.0.48:

Callus \Cal"lus\ (k[a^]l"l[u^]s), n. [L. See Callous.]
   1. (Med.)
      (a) Same as Callosity.
      (b) The material of repair in fractures of bone; a
          substance exuded at the site of fracture, which is at
          first soft or cartilaginous in consistence, but is
          ultimately converted into true bone and unites the
          fragments into a single piece.
          [1913 Webster]

   2. (Hort.) The new formation over the end of a cutting,
      before it puts out rootlets.
      [1913 Webster]

WordNet (r) 3.0 (2006):

callus
    n 1: an area of skin that is thick or hard from continual
         pressure or friction (as the sole of the foot) [syn:
         callosity, callus]
    2: bony tissue formed during the healing of a fractured bone
    3: (botany) an isolated thickening of tissue, especially a stiff
       protuberance on the lip of an orchid
    v 1: cause a callus to form on; "The long march had callused his
         feet"
    2: form a callus or calluses; "His foot callused"