Annulment is a legal procedure for declaring a marriage

Marriage

Marriage is a social union or legal contract between people that creates kinship. It is an institution in which interpersonal relationships, usually intimate and sexual, are acknowledged in a variety of ways, depending on the culture or subculture in which it is found...

 null and void

Void (law)

In law, void means of no legal effect. An action, document or transaction which is void is of no legal effect whatsoever: an absolute nullity - the law treats it as if it had never existed or happened....

. Unlike divorce

Divorce

Divorce is the final termination of a marital union, canceling the legal duties and responsibilities of marriage and dissolving the bonds of matrimony between the parties...

, it is usually retroactive

Ex post facto law

An ex post facto law or retroactive law is a law that retroactively changes the legal consequences of actions committed or relationships that existed prior to the enactment of the law...

, meaning that an annulled marriage is considered to be invalid from the beginning almost as if it had never taken place (though some jurisdictions provide that the marriage is only void from the date of the annulment). For example, this is the case in section 12 of the Matrimonial Causes Act 1973 in England and Wales.

In strict legal terminology, annulment refers only to making a voidable

Voidable

In law, a transaction or action which is voidable is valid, but may be annulled by one of the parties to the transaction. Voidable is usually used in distinction to void ab initio and unenforceable....

marriage null; if the marriage is void ab initio

Void (law)

In law, void means of no legal effect. An action, document or transaction which is void is of no legal effect whatsoever: an absolute nullity - the law treats it as if it had never existed or happened....

, then it is automatically null, although a legal declaration of nullity is required to establish this.