What is Ab Initio?

Legal Definition
Ab initio (/ˌæbˈnɪʃi./ AB-i-NISH-ee-oh) is a Latin term meaning "from the beginning" and is derived from the Latin ab ("from") + initio, ablative singular of initium ("beginning").
-- Wikipedia
Legal Definition
A Latin term meaning "from the beginning." Used to indicate that some fact existed from the start of a relevant time period.
Legal Definition
From the beginning.

2. 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. Bac. Ab. Trespass, B.; 8 Coke, 146 2 Bl. Rep. 1218 Clayt. 44. And if an officer neglect to remove goods attached within a reasonable time and continue in possession, his entry becomes a trespass ab initio. 2 Bl. Rep. 1218. See also as to other cases, 2 Stra. 717 1 H. Bl. 13 11 East, 395 2 Camp. 115 2 Johns. 191; 10 Johns. 253; ibid. 369.

3. 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, Lane, 90 ; Bae. Ab. Trespass, B ; 2 T. It. 166. See generally 1 Chit. PI. 146. 169. 180.
-- Bouviers Law Dictionary
Legal Definition
Latin, for from the first act or meaning from the start.