(Latin: optimus, best)
May be understood as a metaphysical theory, or as an emotional disposition. In philosophy, the theory proposed by Leibnitz, that the world, as the product of a Perfect, All-Good Creator, is the best conceivable world and essentially good.
God foresaw all possible worlds, and was bound to choose the best.
The fact of evil is explained as a necessary product of metaphysical finiteness and moral freedom.
Physical evil is a punishment for abuse of the latter.
In a more moderate sense, optimism holds that the world is good, and the best possible fulfillment of the purpose of the Creator.
It is also an evolutionary theory that the world is constantly tending to a higher and more perfect state (Hegel).
As an emotional tendency it is an habitual and persistent disposition to regard only the bright and cheerful aspects of life, minimizing what is distasteful.
New Catholic Dictionary