What is Adverse Party?

Legal Definition
An adverse party is an opposing party in a lawsuit under an adversary system of law. In general, an adverse party is a party against whom judgment is sought or "a party interested in sustaining a judgment or decree." For example, the adverse party for a defendant is the plaintiff.
-- Wikipedia
Legal Definition
An "adverse party" entitled to notice of appeal is every party whose interest in relation to the judgment or decree appealed from is in conflict with the modification or reversal sought by the appeal; every party interested in sus-, taining the judgment or decree. Harrigan v. Gilchrist, 121 Wis 127, 99 N. W. 909; Moody v. Miller, 24 Or. 179, 33 Pac. 402; Mohr v. Byrne, 132 Cal. 250, 64 Pac. 257; Fitzgerald v. Cross, 30 Ohio St. 444; In re Clarke, 74 Minn. 8, 76 N. W. 790; Herri-man v. Menzies, 115 Cal. 16, 44 Pac. 660, 35 L. R. A. 318, 56 Am. St Rep. 81.
-- Black's Law Dictionary
Legal Definition
Every party who has an interest in conflict with the reversal of a judgment or whose rights might be adversely or injuriously affected by a reversal, irrespective of whether such party is a plaintiff, a defendant, or an intervenor. See 21 Ann. Cas. 1273.
-- Ballentine's Law Dictionary
Legal Definition
The opposing party, or anyone who has interests contrary to yours in a lawsuit.