[Latin, From the beginning; from the first act; from the inception.] An agreement is said to be "void ab initio" if it has at no time had any legal validity. A party may be said to be a trespasser, an estate said to be good, an agreement or deed said to be void, or a marriage or act said to be unlawful, ab initio. Contrasted in this sense withex post facto, or with postea.
The illegality of the conduct or the revelation of the real facts makes the entire situation illegal ab initio (from the beginning), not just from the time the wrongful behavior occurs. A person who enters property under the authority of law but who then by misconduct abuses his or her right to be on the property is considered a trespasser ab initio. If a sheriff enters property under the authority of a court order requiring him to seize a valuable painting, but instead he takes an expensive marble sculpture, he would be a trespasser from the beginning. Since the officer abused his authority, a court would presume that he intended from the outset to use that authority as a cloak from under which to enter the property for a wrongful purpose. This theory, used to correct abuses by public officers, has largely fallen into disuse.