object of hatred; someone viewed with hostility
Almost always used in the pattern 目の敵にする, meaning to treat someone as an enemy or to be hostile toward them. The noun itself refers to the person who is the target of such feelings.
See also: 目の敵にする
He treats me as an enemy.
あの先生は遅刻する生徒を目の敵にする。
That teacher is hostile toward students who are late.