a disposition or tendency to look on the more favorable side of events or conditions and to expect the most favorable outcome.
the belief that good ultimately predominates over evil in the world.
the belief that goodness pervades reality.
the doctrine that the existing world is the best of all possible worlds.

1730–40; < French optimisme < Latin optim(um) (see optimum) + French -isme -ism

antioptimism, noun
overoptimism, noun

1. confidence, hopefulness, cheerfulness.

1, 2. pessimism, cynicism.
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
optimism (ˈɒptɪˌmɪzəm)
1.  the tendency to expect the best and see the best in all things
2.  hopefulness; confidence
3.  the doctrine of the ultimate triumph of good over evil
4.  the philosophical doctrine that this is the best of all possible worlds
[C18: from French optimisme, from Latin optimus best, superlative of bonus good]

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

1782, from Fr. optimisme (1737), from Mod.L. optimum, used by Leibnitz (in Théodicée, 1710) to mean "the greatest good," from L. optimus "the best" (see optimum). The doctrine holds that the actual world is the "best of all possible worlds," in which the creator
accomplishes the most good at the cost of the least evil.
"En termes de l'art, il l'appelle la raison du meilleur ou plus savamment encore, et Theologiquement autant que Géométriquement, le systême de l'Optimum, ou l'Optimisme." [Mémoires de Trévoux, Feb. 1737]
Launched out of philosophical jargon and into currency by Voltaire's satire on it in "Candide." General sense of "belief that good ultimately will prevail in the world" first attested 1841 in Emerson; meaning "tendency to take a hopeful view of things" first recorded 1819 in Shelley.
Online Etymology Dictionary, © 2010 Douglas Harper
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Computing Dictionary

optimism definition

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 Free On-line Dictionary of Computing, © Denis Howe 2010 http://foldoc.org
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Example sentences
But despite the familiar urban problems, there's a goofy, energetic optimism
My heart is filled with optimism for the future of these storied cats.
But the picture is also poignant, in that the boundless optimism it captured
  has been tempered by tragedies and dead ends.
The principal doctor was lost in the imbecilities of a senile optimism.
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