Simple Definition:

Optimistic means having a positive outlook [perspective or way of looking at things] or expecting good things to happen.

Examples:

• Even though she failed the test, she remained optimistic and believed that she would do better next time. (Failed: did not pass)
• John is always optimistic about the future and believes that everything will work out in the end. (Future: time that is yet to come)

Very Simple Definition:

Optimistic means being positive and thinking that good things will happen.

Examples:

• Sarah is optimistic about her chances of winning the game. She believes that she will win.
• Peter is optimistic that he will find a new job soon. He thinks he will get a job offer.