The Collaborative International Dictionary of English v.0.48:

Truism \Tru"ism\, n. [From True.]
   An undoubted or self-evident truth; a statement which is
   pliantly true; a proposition needing no proof or argument; --
   opposed to falsism.
   [1913 Webster]

         Trifling truisms clothed in great, swelling words. --J.
                                                  P. Smith.
   [1913 Webster]

WordNet (r) 3.0 (2006):

truism
    n 1: an obvious truth