Optimism is all about seeing the bright side of things. If you think it’s going to be a great day, even though the bills are due, you lost your wallet, and you forgot your lunch, then you're full of optimism.
Optimism is expecting that the best will happen. It comes from the Latin word optimus, which means "best." Even if you don’t think the very best thing will always happen, if you're generally given to optimism, you think things will get better instead of worse. The opposite of optimism is pessimism, which is the expectation that everything will go wrong, or is wrong.