Definitions of callus:

• noun:   (botany) an isolated thickening of tissue, especially a stiff protuberance on the lip of an orchid
• noun:   bony tissue formed during the healing of a fractured bone
• noun:   an area of skin that is thick or hard from continual pressure or friction (as the sole of the foot)
• verb:   form a callus or calluses

  Example: "His foot callused"

• verb:   cause a callus to form on

  Example: "The long march had callused his feet"