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opponent; rival; adversary
menace; danger; threat; enemy
「敵」 has another reading with different meanings.
彼とは昔から敵対関係にあった。
I was in an adversarial relationship with him.
味方と敵を見分けることが大切だ。
It's important to distinguish allies from enemies.
あの選手は手強い敵だ。
That player is a tough opponent.