The Collaborative International Dictionary of English v.0.48:

Optimism \Op"ti*mism\, n. [L. optimus the best; akin to optio
   choice: cf. F. optimisme. See Option.]
   [1913 Webster]
   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.
      [1913 Webster]

   2. A habitual tendency or a present disposition to take the
      most hopeful view of future events, and to expect a
      favorable outcome even when unfavorable outcomes are
      possible; -- opposed to pessimism.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

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]

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "optimism":
   Leibnizian optimism, Pollyannaism, blitheness, blithesomeness,
   bright outlook, bright side, brightness, buoyancy, cheer,
   cheerful expectation, cheerfulness, cheeriness, cheery vein,
   eupeptic mien, geniality, gladness, gladsomeness, good cheer,
   happiness, hope, hopefulness, idealism, millennialism,
   optimisticalness, perfectibilism, perfectionism,
   philosophical optimism, pleasantness, positivism, radiance,
   rosy expectation, rosy outlook, sanguine expectation,
   sanguine humor, sanguineness, sanguinity, silver lining, sunniness,
   utopianism, winsomeness



The Jargon File (version 4.4.7, 29 Dec 2003):

optimism
 n.

    What a programmer is full of after fixing the last bug and before
    discovering the next last bug. Fred Brooks's book The Mythical Man-Month
    (See Brooks's Law) 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 godmothers.
        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.


The Free On-line Dictionary of Computing (30 December 2018):

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]


The Devil's Dictionary (1881-1906):

OPTIMISM, n.  The doctrine, or belief, that everything is beautiful,
including what is ugly, everything good, especially the bad, and
everything right that is wrong.  It is held with greatest tenacity by
those most accustomed to the mischance of falling into adversity, and
is most acceptably expounded with the grin that apes a smile.  Being a
blind faith, it is inaccessible to the light of disproof -- an
intellectual disorder, yielding to no treatment but death.  It is
hereditary, but fortunately not contagious.