Nullity has 2 possible legal definitions. It can either refer to when something is null, and/or void such as the nullity of a marriage) or it can refer to the circumstances of a void act as when a statute is declared to be a nullity.
1. The state or quality of being null.
2. Something that is null, especially an act having no legal validity.
1: the quality or state of being null
2: an act, proceeding, or contract void of legal effect
n. something which may be treated as nothing, as if it did not exist or never happened. This can occur by court ruling or enactment of a statute. The most common example is a nullity of a marriage by a court judgment.
Lect Law Library
Properly, that which does not exist; that which is not in the nature of things. In a figurative sense it signifies that which has no more effect than if it did not exist.
Properly, that which does not exist; that which is not properly in the nature of things. In a figurative sense, and in law, it means that which has no more effect than if it did not exist, and also the defect which prevents it from having such effect. That which is absolutely void.
It is a rule of law that what is absolutely null produces no effects whatever; as, if a man bad a wife in full life, and both aware of the fact, he married another woman, such second marriage would be nun and without any legal effect.
Nullities have been divided into absolute and relative. Absolute nullities are those which may be insisted upon by any one having an interest in rendering the act, deed or writing null, even by the public authorities, as a second marriage while the former was in full force. Everything fraudulent is null and void. Relative nullities can be invoked only by those in whose favor the law has been established, land, in fact, such power is less a nullity of the act than a faculty which one or more persons have to oppose the validity of the act.