The Collaborative International Dictionary of English v.0.48:

Platitude \Plat"i*tude\, n. [F., from plat flat. See Plate.]
   1. The quality or state of being flat, thin, or insipid; flat
      commonness; triteness; staleness of ideas of language.
      [1913 Webster]

            To hammer one golden grain of wit into a sheet of
            infinite platitude.                   --Motley.
      [1913 Webster]

   2. A thought or remark which is flat, dull, trite, or weak; a
      truism; a commonplace.
      [1913 Webster]

WordNet (r) 3.0 (2006):

platitude
    n 1: a trite or obvious remark [syn: platitude, cliche,
         banality, commonplace, bromide]

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "platitude":
   abstraction, banality, bromide, chestnut, cliche, commonplace,
   commonplace expression, corn, familiar tune, general idea,
   generalization, generalized proposition, glittering generality,
   hackneyed expression, hackneyed saying, inanity, insipidity,
   lieu commun, locus communis, mawkishness, old joke, old saw,
   old song, old story, prosaicism, prosaism, prose, reiteration,
   retold story, rubber stamp, sentimentality, shibboleth,
   stereotyped saying, sweeping statement, tag, tired cliche,
   trite saying, triticism, truism, twice-told tale, vapidity



The Devil's Dictionary (1881-1906):

PLATITUDE, n.  The fundamental element and special glory of popular
literature. A thought that snores in words that smoke.  The wisdom of
a million fools in the diction of a dullard.  A fossil sentiment in
artificial rock.  A moral without the fable.  All that is mortal of a
departed truth.  A demi-tasse of milk-and-mortality.  The Pope's-nose
of a featherless peacock.  A jelly-fish withering on the shore of the
sea of thought.  The cackle surviving the egg.  A desiccated epigram.