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]

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.