1. Something of no importance or worth; nothingness.
2. Acts or things which are legally void or in states of being legally void or invalid; especially, with reference to marriages.
3. A lack of effectiveness or usefulness.
4. Something which may be treated as nothing, as if it did not exist or never happened.

This can take place by a court ruling or an enactment of a statute.

The most common example is a nullity of a marriage by a court judgment.