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"

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "callus":
   Philistinism, armor, benign tumor, callosity, callousness, cancer,
   carcinoma, corn, cyst, excrescence, flintiness,
   formidable defenses, fungosity, fungus, growth, hard heart,
   hard shell, hardenedness, hardheartedness, hardness,
   hardness of heart, heart of stone, imperviousness, induration,
   insensitiveness, insensitivity, intumescence, inuredness,
   malignant growth, metastatic tumor, mole, morbid growth, neoplasm,
   nevus, nonmalignant tumor, obduracy, obdurateness, outgrowth,
   proud flesh, rhinoceros hide, sarcoma, stoniness, thick skin,
   tumor, verruca, wart, wen