callus

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A localized thickening and enlargement of the horny layer of the skin.
• noun The hard bony tissue that develops around the ends of a fractured bone during healing.
• noun Undifferentiated tissue that develops on or around an injured or cut plant surface or in tissue culture.
• noun The hardened, sometimes sharp base of the spikelet of certain grasses.
• intransitive verb To form or develop such hardened tissue.

from The Century Dictionary.

• noun In anatomy: Hard skin; a callosity
• noun A new growth of osseous tissue between and around the extremities of fractured bones, serving to unite them.
• noun In botany, any unusually hard excrescence upon a plant; also, the thickening of the substance of the perforated septa between sieve-cells, and the close cellular structure which is formed over wounds, by which the inner tissues are protected and healing is effected.
• noun In horticulture, the cap or thickening formed over the end of a cutting before it sends forth rootlets.
• noun In conchology, a callosity or indurated thickening of a shell by the deposit of some hard substance different from the rest of the shell.

from the GNU version of the Collaborative International Dictionary of English.

• noun Same as callosity
• noun 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.
• noun (Hort.) The new formation over the end of a cutting, before it puts out rootlets.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A hardened area of the skin (especially on the foot or hand) caused by repeated friction, wear or use.
• noun The material of repair in fractures of bone; a substance exuded at the site of fracture, which is at first soft or cartilaginous in consistency, but is ultimately converted into true bone and unites the fragments into a single piece.
• noun botany The new formation over the end of a cutting, before it puts out rootlets.
• verb intransitive To form such hardened tissue

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

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