The term optimist was originally applied to those who adhered to the philosophy of optimism. This declared that the world was the best of all possible worlds, as it was created by God. Therefore, even what appeared bad was actually part of God's perfection.
The term came to be used as meaning someone who looks for the positive side of a situation and anticipates the most positive outcome of a venture.
An optimist is one who looks at a glass of water that is half-full and half-empty, and focuses on how it is half-full. A pessimist focuses on how it is half-empty.