The Collaborative International Dictionary of English v.0.48:

Utilitarianism \U*til`i*ta"ri*an*ism\, n.
   1. The doctrine that the greatest happiness of the greatest
      number should be the end and aim of all social and
      political institutions. --Bentham.
      [1913 Webster]

   2. The doctrine that virtue is founded in utility, or that
      virtue is defined and enforced by its tendency to promote
      the highest happiness of the universe. --J. S. Mill.
      [1913 Webster]

   3. The doctrine that utility is the sole standard of
      morality, so that the rectitude of an action is determined
      by its usefulness.
      [1913 Webster]

WordNet (r) 3.0 (2006):

utilitarianism
    n 1: doctrine that the useful is the good; especially as
         elaborated by Jeremy Bentham and James Mill; the aim was
         said to be the greatest happiness for the greatest number