The Collaborative International Dictionary of English v.0.48:

Medallion \Me*dal"lion\, n. [F. m['e]daillion, It. medaglione,
   augm. of medaglia. See Medal.]
   [1913 Webster]
   1. A large medal or memorial coin.
      [1913 Webster]

   2. A circular or oval (or, sometimes, square) tablet bearing
      a figure or figures represented in relief.
      [1913 Webster]

WordNet (r) 3.0 (2006):

medallion
    n 1: any of various large ancient Greek coins
    2: a circular helping of food (especially a boneless cut of
       meat); "medallions of veal"
    3: an emblem indicating that a taxicab is registered
    4: an award for winning a championship or commemorating some
       other event [syn: decoration, laurel wreath, medal,
       medallion, palm, ribbon]

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "medallion":
   Bronze Star Medal, Croix de Guerre, Distinguished Conduct Medal,
   Distinguished Flying Cross, Distinguished Service Cross,
   Distinguished Service Medal, Distinguished Service Order,
   Distinguished Unit Citation, Medaille Militaire, Military Cross,
   Navy Cross, Unit Citation, Victoria Cross, anaglyph, bas-relief,
   boss, cameo, cameo glass, cavo-rilievo, cut glass, embossment,
   glyph, glyptograph, high relief, intaglio, intaglio rilevato,
   intaglio rilievo, low relief, mask, medal, military medal, order,
   plaquette, relief, relievo, sculptured glass, service medal,
   war medal