The Collaborative International Dictionary of English v.0.48:

decoration \dec`o*ra"tion\ (d[e^]k`[-o]*r[=a]"sh[u^]n), n. [LL.
   decoratio: cf. F. d['e]coration.]
   1. The act of adorning, embellishing, or honoring;
      ornamentation.
      [1913 Webster]

   2. That which adorns, enriches, or beautifies; something
      added by way of embellishment; ornament.
      [1913 Webster]

            The hall was celebrated for . . . the richness of
            its decoration.                       --Motley.
      [1913 Webster]

   3. Specifically, any mark of honor to be worn upon the
      person, as a medal, cross, or ribbon of an order of
      knighthood, bestowed for services in war, great
      achievements in literature, art, etc.
      [1913 Webster]

WordNet (r) 3.0 (2006):

decoration
    n 1: something used to beautify [syn: decoration, ornament,
         ornamentation]
    2: an award for winning a championship or commemorating some
       other event [syn: decoration, laurel wreath, medal,
       medallion, palm, ribbon]
    3: the act of decorating something (in the hope of making it
       more attractive)

Moby Thesaurus II by Grady Ward, 1.0:

136 Moby Thesaurus words for "decoration":
   abstract art, accolade, adjunct, adornment, armory, arrangement,
   art, art form, artist, arts and crafts, arts of design, award,
   badge, badge of office, badges, baton, bays, beautification,
   beauty treatment, blazonry, blue ribbon, bonus, brassard, button,
   calligraphy, cap and gown, cave art, ceramics, chain,
   chain of office, class ring, cockade, collar, color,
   color patterns, colors, cordon, cordon bleu, cross, decor,
   decoration of honor, design, designing, distinction, dress, eagle,
   elaboration, embellishment, emblazonment, emblazonry, emblems,
   embroidery, engraving, ensigns, etching, extra,
   extra added attraction, extra dash, facial, fasces, figurehead,
   filigree, filling, fillip, fine arts, fleur-de-lis, flourish,
   flower arrangement, folk art, frill, furniture arrangement,
   garnish, garnishment, garniture, garter, gold star, grand cordon,
   graphic arts, hairdressing, hammer and sickle, heraldry,
   illumination, insignia, kudos, lagniappe, lapel pin, laurel,
   laurels, livery, mace, manicure, mantle, markings, medal,
   mortarboard, old school tie, order, ornament, ornamentation,
   padding, photography, pin, plastic art, premium, prettification,
   primitive art, red ribbon, regalia, riband, ribbon, ring, rose,
   school ring, sculpture, shamrock, sigillography,
   skull and crossbones, something extra, sphragistics, staff, star,
   stuffing, superaddition, swastika, tartan, the arts, thistle, tie,
   trim, trimming, twist, uniform, verge, wand, window dressing,
   wrinkle