If you defeat someone, you win a victory over them in a battle, game, or contest.

If a proposal or motion in a debate is defeated, more people vote against it than for it.

If a task or a problem defeats you, it is so difficult that you cannot do it or solve it.

To defeat an action or plan means to cause it to fail.

Defeat is the experience of being beaten in a battle, game, or contest, or of failing to achieve what you wanted to.