Lat. 'from the beginning'. Usually used in reference to contracts, estates, trespasses and deeds to indicate their existence or validity relates back to their creation like an (unlawful) marriage 'ab initio' meaning the marriage was void during its entire alleged existence.
When a man enters upon lands or into the house of another by authority of law, and afterwards abuses that authority, he becomes a trespasser ab initio. And if an officer neglects to remove goods attached within a reasonable time and continues in possession, his entry becomes a trespass ab initio. But in case of an authority in fact, to enter, an abuse of such authority will not, in general, subject the party to an action of trespass.