/ ˈædvəsəri; NAmE -vərseri/ noun
( pl. -ies ) ( formal ) a person that sb is opposed to and competing with in an argument or a battle
SYN opponent
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WORD ORIGIN
Middle English : from Old French adversarie , from Latin adversarius opposed, opponent, from adversus against, opposite, past participle of advertere , from ad- to + vertere to turn.