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