nullity of marriage, in law, an unlawful marriage that is either void or voidable because of conditions existing at the time of the marriage. A bigamous or incestuous marriage, for example, is void, and there is no need to bring a suit to obtain a decree declaring it void. However, a decree is necessary to annul voidable marriages. Grounds, generally specified by statute, include fraud or duress preventing legal consent to the marriage and sexual impotence of one spouse that existed at the time the marriage was contracted and that was unknown to the other spouse. The results of nullity of marriage are different from those of divorce. A decree of nullity declares, in effect, that the parties never were married, and at one time it absolved them from all obligations to each other. Today, however, statutes in some states have mitigated the results of annulment, e.g., by making children of the void marriage legitimate and by permitting alimony. See husband and wife.