The noun adversary means someone you are competing with, or arguing or fighting against.

Synonyms are opponent, foe, rival, or antagonist.

The word origins from Anglo-French (13th century) adverser and Old French adversarie (12th century) meaning “hostile opponent, enemy”. It also sources directly from Latin adversarius meaning “an opponent, rival, enemy”, noun use of adjective meaning “opposite, hostile, contrary”, literally “turned toward one”, from adversus meaning “turned against, turned toward, fronting, facing”, figuratively “hostile, adverse, unfavorable”. It comes from ad meaning “to” plus vertere meaning “to turn, turn back, be turned, convert, transform, translate, be changed”.

The secret to a lasting relationship is to never treat your husband or wife as an adversary.

The news was designed to incite the corrupt politician who is known as the people’s adversary.

Before the combat, he already knew his adversary’s overall capability.