Annulment is a legal procedure within secular and religious legal systems for declaring a marriage null and void. Unlike divorce, it is usually retroactive, 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 legal terminology, an annulment makes a void marriage or a voidable marriage null.

A void marriage is a marriage that is unlawful or invalid under the laws of the jurisdiction where it is entered. A void marriage is "one that is void and invalid from its beginning. It is as though the marriage never existed and it requires no formality to terminate." A marriage that is entered into in good faith, but that is later found to be void, may be recognized as a putative marriage and the spouses as putative spouses, with certain rights granted by statute or common law, notwithstanding that the marriage itself is void.

Void marriages are distinct from those marriages that can be canceled at the option of one of the parties, but otherwise remain valid. Such a marriage is voidable, meaning that it is subject to cancellation through annulment if contested in court.

A voidable marriage (also called an avoidable marriage) is a marriage that can be canceled at the option of one of the parties through annulment. The marriage is valid but is subject to cancellation if contested in court by one of the parties to the marriage.

A voidable marriage is contrasted with a void marriage, which is one that is on its face unlawful and therefore legally has no effect, whether or not one of the parties challenges the marriage.