To declare something nonexistent is to make it void. A void can happen on any contract including marriage (known as annulments). A contract will automatically be considered to be void if either party misrepresent themselves within the context of the agreement or do not comply with all of the terms set within it. Declaring something invalid or creating a situation where a contract is not legally binding is called void.
The word void is Middle English in origin and means hollow or empty. It has been taken to mean nonexistent within the context of law. It is relied upon when a contract ceases to operate as intended. A court can also void any contract which does not comply with law even if both sides have kept to the terms set forth generally.