ab initio
adverb
ab in·i·tio ˌab-ə-ˈni-shē-ˌō 
: from the beginning
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Starting Things With Ab Initio
We'll tell you right from the beginning where ab initio comes from. This adverb was adopted at the beginning of the 17th century directly from Latin, where it translates as "from the beginning." (Initio is a form of the noun initium, meaning "beginning," which gave rise to such English words as initial, initiate, and initiative.) Ab initio most frequently appears in legal contexts, but it is not surprising to find it used outside of the courtroom. The phrase is also used as an adjective meaning "starting from or based on first principles" (as in "predicted from ab initio calculations").
Example Sentences
Recent Examples on the Web What McConnell needed to do, for his argument to establish its conclusions, was to provide independent grounds of some kind for ruling out Interpretation B ab initio. 
Jason Lee Steorts, National Review, 15 May 2021 Moreover, the Executive Order exceeds the President’s authority and is therefore ultra vires and void ab initio. Eriq Gardner, Billboard, 4 Sep. 2020
Jason Lee Steorts, National Review, 15 May 2021 Moreover, the Executive Order exceeds the President’s authority and is therefore ultra vires and void ab initio. Eriq Gardner, Billboard, 4 Sep. 2020 Word History
Etymology
Latin ab initiō
First Known Use
1600, in the meaning defined above
Legal Definition
ab initio
adverb
ab ini·tio ˌab-ə-ˈni-shē-ˌō, ˌäb-i-ˈnē-tē-ˌō
: from the beginning
a contract found to be void ab initio