Annulment is a legal procedure 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 voidable marriage null. If a marriage is void ab initio, then it is automatically null, although a declaration of nullity is required to establish this. The process of obtaining a declaration of nullity is similar to an annulment process. Despite its retroactive nature, children born before an annulment are considered legitimate in many countries.
Annulments are closely associated with the Catholic Church, which does not permit divorce, teaching that marriage is a lifelong commitment which cannot be dissolved through divorce, but can be annulled if invalidly entered into.
Read more about Annulment: Annulment in The Catholic Church, Annulment in New York State, Annulment in The State of Nevada, Multiple Annulments