A more thorough explanation:

A nullity suit is a legal action taken to declare a marriage invalid. This can happen if the marriage was never valid to begin with or if it was voided by a court order.

For example, if two people who are closely related get married, their marriage is automatically void and does not need to be ended through a formal process. However, if a couple gets married and later discovers that one of them was already married at the time of the wedding, they may need to file a nullity suit to have the marriage declared invalid.

Nullity suits are also sometimes called annulment suits. They are different from divorce suits, which end a valid marriage.