cameo

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

• noun A gem or shell carved in relief, especially one in which the raised design and the background consist of layers of contrasting colors.
• noun The technique of carving in this way.
• noun A medallion with a profile cut in raised relief.
• noun A brief vivid portrayal or depiction.
• noun A brief appearance of a prominent actor or celebrity, as in a single scene of a motion picture.
• intransitive verb To make into or like a gem or shell carved in relief.
• intransitive verb To portray in sharp, delicate relief, as in a literary composition.
• intransitive verb To make a brief appearance, as in a film.

from The Century Dictionary.

• noun An engraving in relief upon a gem, a hard stone of moderate size, or a similar material, or the object itself so engraved, as distinguished from an intaglio; specifically, such an engraving upon a stone or a shell having two or three layers differing in color, such as an onyx, agate, etc., and so treated as to utilize the effect of the variety of coloring.
• noun Hence Raised or anaglyphic work in art on a miniature scale; specifically, the art of engraving small figures in relief: opposed to intaglio: as, a stone or shell cut in cameo; a vase ornamented in cameo.

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

• noun A carving in relief, esp. one on a small scale used as a jewel for personal adornment, or like.
• noun (Zoöl.) a large, marine, univalve shell, esp. Cassis cameo, Cassis rua, and allied species, used for cutting cameos. See Quern conch.

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

• noun A piece of jewelry, etc., carved in relief.
• noun A single very brief appearance by a prominent celebrity in a movie or song.
• verb To appear in a cameo role.

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

• noun engraving or carving in low relief on a stone (as in a brooch or ring)