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

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "truism":
   a priori truth, abstraction, aphorism, apothegm, axiom, banality,
   brocard, bromide, cliche, commonplace, dictate, dictum, formula,
   general idea, generalization, generalized proposition,
   glittering generality, gnome, golden rule, gospel,
   hackneyed expression, law, lieu commun, locus communis, maxim,
   moral, platitude, postulate, principium, principle, proposition,
   prosaicism, prosaism, rubber stamp, rule, self-evident truth,
   settled principle, shibboleth, sweeping statement, tag, theorem,
   tired cliche, truth, universal truth, verity