The Collaborative International Dictionary of English v.0.48:

Optimist \Op"ti*mist\, n. [Cf. F. optimiste.]
   [1913 Webster]
   1. (Metaph.) One who holds the opinion that all events are
      ordered for the best.
      [1913 Webster]

   2. One who looks on the bright side of things, or takes
      hopeful views; one who experiences optimism[2]; -- opposed
      to pessimist.
      [1913 Webster]

WordNet (r) 3.0 (2006):

optimist
    n 1: a person disposed to take a favorable view of things [ant:
         pessimist]

Moby Thesaurus II by Grady Ward, 1.0:

20 Moby Thesaurus words for "optimist":
   Leibnizian optimist, Pollyanna, aspirant, aspirer, chiliast,
   dreamer, hopeful, hoper, idealist, irrepressible optimist,
   millenarian, millennialist, millennian, perfectibilist,
   perfectibilitarian, perfectionist, philosophical optimist,
   positivist, ray of sunshine, utopian



The Devil's Dictionary (1881-1906):

OPTIMIST, n.  A proponent of the doctrine that black is white.
    A pessimist applied to God for relief.
    "Ah, you wish me to restore your hope and cheerfulness," said God.
    "No," replied the petitioner, "I wish you to create something that
would justify them."
    "The world is all created," said God, "but you have overlooked
something -- the mortality of the optimist."