ab initioEnglishOxford New American Dictionaryab initio /ˌab əˈniSHēˌō, ˌæb əˈnɪʃiˌoʊ / formal or Law ▸ adverb from the beginning: the agreement should be declared void ab initio. ▸ adjective [attributive] starting from the beginning: he was instructing ab initio pilots. – ORIGIN Latin.