(Hebrews satan), an opponent or foe (1 Kings 5:4; 11:14, 23, 25; Luke 13:17); one that speaks against another, a complainant (Matthew 5:25; Luke 12:58); an enemy (Luke 18:3), and specially the devil (1 Peter 5:8).

Noah Webster's New International Dictionary of the English Language

1. (n.) One who is turned against another or others with a design to oppose or resist them; a member of an opposing or hostile party; an opponent; an antagonist; an enemy; a foe.

2. (a.) Opposed; opposite; adverse; antagonistic.

3. (a.) Having an opposing party; not unopposed; as, an adversary suit.