In being; actually existing; -- distinguished from in posse, or in potentia, which denote that a thing is not, but may be.
Source: Webster's dictionaryHe attempted to clarify Aquinas's Five Ways (in the Summa Theologica ) by making a distinction between in fieri causes and in esse causes. Source: Internet
In fieri is generally translated as "becoming", while in esse is generally translated as "in essence". Source: Internet