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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]