A proposition in law that a court's jurisdiction, a certain document which purports to affect legal rights, or an act which purports to affect legal rights, is or was nul and void from the start, from its beginning, because of some vitiating element.
Typically, documents or acts which are ab initio cannot be fixed and where jurisdiction, a document or an act is so declared at law to be void ab initio, the parties are returned to their respective positions at the time of the ab initio event.
Ab initio (void) is often contrasted with voidable, such documents which become void only as of the date of the judicial declaration to this effect and not, as with ab initio, as if they never existed.
An insurer facing a claim from an insured who had deceived the insurer on a material fact, would claim that the insurance contract was void ab initio; that it was null and void from the get-go; that since there was at law no enforceable contract, the insurer ought not to have to pay.