The Collaborative International Dictionary of English v.0.48:

Ostentation \Os`ten*ta"tion\, n. [L. ostentatio: cf. F.
   ostentation.]
   1. The act of ostentating or of making an ambitious display;
      unnecessary show; pretentious parade; -- usually in a
      detractive sense. "Much ostentation vain of fleshly arm."
      --Milton.
      [1913 Webster]

            He knew that good and bountiful minds were sometimes
            inclined to ostentation.              --Atterbury.
      [1913 Webster]

   2. A show or spectacle. [Obs.] --Shak.
      [1913 Webster]

   Syn: Parade; pageantry; show; pomp; pompousness; vaunting;
        boasting. See Parade.
        [1913 Webster]

WordNet (r) 3.0 (2006):

ostentation
    n 1: a gaudy outward display [syn: ostentation, fanfare,
         flash]
    2: lack of elegance as a consequence of being pompous and puffed
       up with vanity [syn: ostentation, ostentatiousness,
       pomposity, pompousness, pretentiousness, puffiness,
       splashiness, inflation]
    3: pretentious or showy or vulgar display

Moby Thesaurus II by Grady Ward, 1.0:

162 Moby Thesaurus words for "ostentation":
   Barnumism, acting, affectation, appearance, arabesque, arrantness,
   attitudinizing, baroque, baroqueness, bedizenment, big talk,
   blatancy, bluff, bluffing, boldness, cheating, chinoiserie, color,
   coloring, conspicuousness, convolution, deception, delusion,
   demonstration, disguise, display, dissemblance, dissembling,
   dissimulation, elaborateness, elegance, enactment, exhibit,
   exhibition, exhibitionism, exposition, exposure, facade, face,
   fakery, faking, false air, false front, false show, falsity,
   fanciness, feigning, feint, fineness, flagrance, flagrancy,
   flamboyance, flashiness, flatulence, flatulency, flaunting,
   floridity, floridness, flourish, floweriness, four-flushing, fraud,
   front, fulsomeness, garishness, gaudiness, gilt, gloss,
   grandiloquence, grandioseness, grandiosity, high relief,
   high-flown diction, humbug, humbuggery, imposture, inflatedness,
   inflation, lexiphanicism, loftiness, luridness, luxuriance,
   luxuriousness, magniloquence, masquerade, mere rhetoric,
   meretriciousness, moresque, noticeability, notoriety,
   notoriousness, obtrusiveness, opening, orotundity,
   ostentatious complexity, outstandingness, outward show,
   overelaborateness, overelegance, overornamentation, parade,
   performance, platitudinous ponderosity, playacting,
   polysyllabic profundity, pomposity, pompous prolixity, pompousness,
   pontification, pose, posing, posture, presentation, presentment,
   pretense, pretension, pretentiousness, pretext, production,
   projection, prominence, pronouncedness, prose run mad,
   representation, retrospective, rhetoric, rhetoricalness, richness,
   rococo, salience, saliency, seeming, semblance, sensationalism,
   sententiousness, sham, show, showiness, showing, simulacrum,
   simulation, speciousness, stiltedness, strikingness, strong relief,
   swelling utterance, swollen phrase, swollenness, tall talk,
   tortuosity, tortuousness, tumidity, tumidness, turgescence,
   turgidity, unfolding, unfoldment, unveiling, varnish,
   varnishing day, vernissage, window dressing