defeat 作动词是「击败（某人/某军队/某对手）」（defeat the enemy = 击败敌人；defeat a bill = 否决议案）；作名词是「失败/挫败」（suffer a defeat = 遭受失败；admit defeat = 承认失败）。关键区分：defeat 是「主动打败对方」，不是「自己失败」——「我输了」说 I was defeated / I lost，不说 I defeated。
释义
1
v.击败；战胜（对手/困难）
The home team defeated their rivals 3–1 in the championship final.
主队在冠军决赛中以3比1击败了对手。
2
v.使受挫；使失败（问题/困难「难倒」某人）
The complexity of the problem defeated even the most experienced engineers.
这个问题的复杂性甚至难倒了最有经验的工程师。
3
n.失败；挫败
The team accepted their defeat gracefully and vowed to return stronger.
球队优雅地接受了失败，并发誓以更强的状态回归。
语感
admit defeat = 承认失败；defy defeat = 不认输——两个高频固定搭配
击败（动）难倒/使失败（动）失败（名）
名词搭配：admit / accept defeat（承认失败）；suffer a defeat（遭受失败）；a crushing / heavy defeat（惨败）。动词比喻用法：the task defeated him = 这项任务难倒了他（defeat 以任务/问题为主语，人为宾语）。区别：defeat（使对手输）vs lose（自己输）vs fail（失败）——三者方向不同，defeat 永远是「我打败了它/他」。
陷阱
⚠️defeat 是「使……输」，主语不能是「失败者」
✗
Our team defeated in the final.
我们队在决赛中输了。
defeat 作动词是及物动词，必须有宾语（打败了谁）；自己「输了」用 was defeated / lost in the final
✓
Our team was defeated in the final. / Our team lost in the final.
我们队在决赛中落败了。
⚠️defeat the purpose = 违背初衷（固定表达）
Buying a gym membership and never going defeats the purpose.
✗
这里 defeat 是「击败」的含义
✓
defeat the purpose 是固定短语，意为「违背（某事的）初衷/使目的落空」，是地道的英语表达