The Collaborative International Dictionary of English v.0.48:

Cameo \Cam"e*o\ (k[a^]m"[-e]*[-o]), n.; pl. Cameos
   (k[a^]m"[-e]*[=o]z). [It. cammeo; akin to F. cam['e]e,
   cama["i]eu, Sp. camafeo, LL. camaeus, camahutus; of unknown
   origin.]
   A carving in relief, esp. one on a small scale used as a
   jewel for personal adornment, or like.
   [1913 Webster +PJC]

   Note: Most cameos are carved in a material which has layers
         of different colors, such stones as the onyx and
         sardonyx, and various kinds of shells, being used. The
         classical cameos made in Italy are carved on a seashell
         (see cameo conch, below), having an olive figure
         carved from the inner layer of the shell in relief on
         the white background of the outer layer of the shell.
         [1913 Webster]

   Cameo conch (Zool.), a large, marine, univalve shell, esp.
      Cassis cameo, Cassis rua, and allied species, used for
      cutting cameos. See Quern conch.
      [1913 Webster]

WordNet (r) 3.0 (2006):

cameo
    n 1: engraving or carving in low relief on a stone (as in a
         brooch or ring)

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "cameo":
   anaglyph, bas-relief, boss, cameo glass, catalog, cataloging,
   cavo-rilievo, character, character sketch, characterization,
   cut glass, delineation, depiction, description, details,
   embossment, evocation, glyph, glyptograph, graphic account,
   high relief, image, imagery, impression, intaglio,
   intaglio rilevato, intaglio rilievo, itemization, limning,
   low relief, mask, medal, medallion, particularization, photograph,
   picture, plaquette, portrait, portraiture, portrayal, profile,
   relief, relievo, rendering, rendition, representation,
   sculptured glass, sketch, specification, vignette,
   vivid description, word painting