More importantly, however, Frege was the first to claim that a properly formed definition had to have two important metatheoretical properties.  Let us call the new, defined symbol introduced in a definition the definiendum, and the term that is used to define the new term the definiens.  Then Frege was the first to suggest that proper definitions have to be both eliminable (a definendum must always be replaceable by its definiens in any formula in which the former occurs) and conservative (a definition should not make it possible to prove new relationships among formulas that were formerly unprovable).  Concerning one of his definitions in the Begriffsschrift (§24), Frege writes: 