The main puzzle about theoretical definitions is that nothing seems to decide which assumptions contribute to such definitions and which do not. I argue that theoretical definitions are indeed imprecise, but that this does not normally matter, since the definitional imprecision does not normally produce indeterminacy of referential value. Sometimes, however, the definitional imprecision is less benign, and does generate referential indeterminacy. In these special cases, but not otherwise, it is necessary to refine the term's definition.