 Some stipulative definitions are nothing but mere devices of abbreviation (e.g., the definitions governing the omission of parentheses in formulas; see Church 1956, §11). However, many stipulative definitions are not of this kind; they introduce meaningful items into our discourse. Thus, definition (4) renders \(G\) a meaningful unary predicate: \(G\) expresses, in virtue of (4), a particular concept. In contrast, under stipulation (6), \(G\) is not a meaningful predicate and expresses no concept of any kind. But what is the source of the difference? Why is (4) legitimate, but not (6)? More generally, when is a definition legitimate? What requirements must the definiens fulfill? And, for that matter, the definiendum? Must the definiendum be, for instance, atomic, as in (3) and (4)? If not, what restrictions (if any) are there on the definiendum?