From The Art and Popular Culture Encyclopedia
Optimism is a mental attitude that interprets situations and events as being best (optimized), meaning that in some way for factors that may not be fully comprehended, the present moment is in an optimum state. The concept is typically extended to include the attitude of hope for future conditions unfolding as optimal as well. The more broad concept of optimism is the understanding that all of nature, past, present and future, operates by laws of optimization along the lines of Hamilton's principle of optimization in the realm of physics. This understanding, although criticized by counter views such as pessimism, idealism and realism, leads to a state of mind that believes everything is as it should be, and that the future will be as well. A common idiom used to illustrate optimism versus pessimism is a glass with water at the halfway point, where the optimist is said to see the glass as half full, but the pessimist sees the glass as half empty.
The word is originally derived from the Latin optimum, meaning "best." Being optimistic, in the typical sense of the word, ultimately means one expects the best possible outcome from any given situation. This is usually referred to in psychology as dispositional optimism.