An optimist is a person who engages in positive thinking, or "looking on the bright side" of things. Optimism is the outlook that good things will happen, even if the situation may not appear totally positive at the moment. An optimist is the opposite of a pessimist.

While a pessimist focuses on the negative aspects of a situation, the optimist focuses on the positive aspects. The half-empty/half-full glass is a classic example of the difference between optimism and pessimism. The optimist will see the glass as half-full as he or she will appreciate that there is still half a glass of something left. The pessimist, on the other hand, will see the same glass as half-empty as he or she will concentrate on what is missing or no longer in the glass.

Sir Winston Churchill was a self-described optimist. He said "For myself, I am an optimist — it does not seem to be much use being anything else." Optimists tend to be more open to possibilities as, unlike pessimists, they tend to not discount something that may exist. Helen Keller is noted as saying that "No pessimist ever discovered the secret of the stars or sailed an uncharted land or opened a new doorway into the human spirit."

However, optimists have been criticized for putting so much of a positive spin on things that they may be avoiding reality. Expressions such as “Seeing the world through rose-colored glasses" or “having your head in the sand" can be said of optimists who seem to think something is positive, when perhaps it is not. Daniel J. Keardon is credited with the comment that "In the long run, the pessimist may be proved right, but the optimist has a better time on the trip."

Yet, optimists are not usually passive in wanting situations to be the best they can be, but are said to tend to take a proactive approach in making things better. Studies show that optimists have a lower risk for depression than pessimists and also tend to have better general health. Optimists are also said to have better coping skills as they tend to realize what they can control and what they can’t in helping to make their lives more positive.