Source : Webster's Revised Unabridged Dictionary (1913)

Callus \Cal"lus\, 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.

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

Source : WordNet®

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}]
     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"
     [also: {calli} (pl)]