Nullity is the fact, or the legal state, that a marriage or a contract has no legal force. It is the noun that sits beside null. If a court grants a decree of nullity, it is saying the marriage was not valid in law — which is not quite the same as a divorce. A divorce ends a marriage that the law once accepted. Nullity says the marriage, or the contract, did not have legal force, so in the eyes of the law it does not stand. You will meet this in legal English and in news about family courts, not in casual talk.

（婚姻或契約的）無效。婚姻無效和離婚不一樣。