An annulment is a declaration by a Church tribunal (a Catholic church court) that a marriage thought to be valid according to Church law actually fell short of at least one of the essential elements required for a binding union. 

For a Catholic marriage to be valid, it is required that: (1) the spouses are free to marry; (2) they are capable of giving their consent to marry; (3) they freely exchange their consent; (4) in consenting to marry, they have the intention to marry for life, to be faithful to one another and be open to children; (5) they intend the good of each other; and (6) their consent is given in the presence of two witnesses and before a properly authorized Church minister. Exceptions to the last requirement must be approved by Church authority.