Source : Webster's Revised Unabridged Dictionary (1913)

Optimism \Op"ti*mism\, n. [L. optimus the best; akin to optio
   choice: cf. F. optimisme. See {Option}.]
   1. (Metaph.) The opinion or doctrine that everything in
      nature, being the work of God, is ordered for the best, or
      that the ordering of things in the universe is such as to
      produce the highest good.

   2. A disposition to take the most hopeful view; -- opposed to
      {pessimism}.

Source : WordNet®

optimism
     n 1: the optimistic feeling that all is going to turn out well
          [ant: {pessimism}]
     2: a general disposition to expect the best in all things [ant:
         {pessimism}]

Source : Free On-Line Dictionary of Computing

optimism
     
        What a programmer is full of after fixing the last bug and
        just before actually discovering the *next* last bug.  Fred
        Brooks's book "{The Mythical Man-Month}" contains the
        following paragraph that describes this extremely well.
     
        All programmers are optimists.  Perhaps this modern sorcery
        especially attracts those who believe in happy endings and
        fairy god-mothers.  Perhaps the hundreds of nitty frustrations
        drive away all but those who habitually focus on the end goal.
        Perhaps it is merely that computers are young, programmers are
        younger, and the young are always optimists.  But however the
        selection process works, the result is indisputable: "This
        time it will surely run," or "I just found the last bug.".
     
        See also {Lubarsky's Law of Cybernetic Entomology}.
     
        [{Jargon File}]