The word "adversary" is used 22 times in the King James Bible, nine times in the NIV, and 16 times in the NRSV. It has various meanings:
an opponent or foe (1 Kings 5:4; 11:14,23,25; Luke 13:17)
one that speaks against another, a complainant (Matt. 5:25; Luke 12:58)
an enemy (Luke 18:3)
the devil (1 Pet. 5:8 - Greek: "antidikos")