The Collaborative International Dictionary of English v.0.48:

Pessimism \Pes"si*mism\, n. [L. pessimus worst, superl. of pejor
   worse: cf. F. pessimisme. Cf. Impair.]
   1. (Metaph.) The opinion or doctrine that everything in
      nature is ordered for or tends to the worst, or that the
      world is wholly evil; -- opposed to optimism.
      [1913 Webster]

   2. A disposition to take the least hopeful view of a
      situation.
      [1913 Webster]

WordNet (r) 3.0 (2006):

pessimism
    n 1: the feeling that things will turn out badly [ant:
         optimism]
    2: a general disposition to look on the dark side and to expect
       the worst in all things [ant: optimism]

The Devil's Dictionary (1881-1906):

PESSIMISM, n.  A philosophy forced upon the convictions of the
observer by the disheartening prevalence of the optimist with his
scarecrow hope and his unsightly smile.