From 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]
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