Learner's definition of OPPONENT
a person, team, group, etc., that is competing against another in a contest
She is a formidable opponent in the race for senator.
The team's opponents have not lost a game this season.
He knocked out his opponent in the third round.
a person, group, etc., that is against something (such as an action, law, or system)
someone or something that does not want something to exist, be done, etc.
— often + of