prep. Latin phrase meaning "from the start"; literal meaning being something done 'from scratch'. In legal parlance it stands from: 1.) if any legal agreement is void ab initio then it stands null and void from the very beginning of its intended existence and not just from the instant its declared as void. 2.) if a person enters onto someone's private property (real estate) by authority of law but later maltreats that authority then he becomes a trespasser ab initio.