A proposition in law that as within 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 a Court declares a document or an act 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, ever.
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.
A contract that is found later to be illegal, never really a contract to begin with, it is said to be void ab initio. A contract that compels someone to do something that it ilegal is void ab initio, from the get go. It never was, in law, a contract as it were.