Defeat is the opposite of victory. When you lose, you suffer defeat. When you win, you defeat your enemy.
This is a word that's all about losing. In World War II, America and the Allies defeated the Axis powers: we beat them. In basketball, Michael Jordan usually defeated his opponents. Any loss can be called a defeat. When you're disappointed or think that nothing is going right, you feel defeated. If you're determined to win, you could say, "I won't accept defeat!" Some people are so stubborn that even though they've obviously been defeated, they won't admit defeat.