In a contested divorce, a spouse is an adversary. If a divorce is uncontested, none of the issues -- such as custody, child support or alimony -- are litigated. A spouse may start off as an adversary and upon reaching an agreement and signing a settlement agreement, the agreement dissolves the adversarial relationship.
In some jurisdictions and divorce actions, the parties in a divorce are called the appellant and respondent, not the plaintiff and defendant.