Defining terminology and nomenclature in such a manner that important features and properties of a system or phenomena for which a description is sought are succinctly captured is a task fraught with difficulty, in particular when dealing either with complex or subtle issues. The recent exchanges in the literature regarding the terminology applied to co-crystals and solvates (pseudopolymorphs) is a case in point. The result of these discussions is that there is still no consistent usage, and perhaps there will never be. Nonetheless, it is one thing to disagree about nomenclature and quite another to misuse it and it is one of these misuses that this paper will broach, namely the designation of different crystalline phases of one and the same protein as polymorphs. Based upon the crystallization behavior and the structural properties of the resulting protein crystals, it will be shown that the term is anything but appropriate. The nature of protein crystals will be discussed here with reference to structural aspects and crystal types of small molecules and ionic crystals in order to identify the similarities to crystalline proteins as well as any differences. It will become clear that protein crystals are always solvates and, as far as experimental evidence allows to make a clear statement, often solvates of salts. Quite clearly, however, the use of the term polymorph to describe different crystal modifications cannot be justified unless it can be clearly shown that the modifications it is applied to have exactly the same chemical composition. In practice, this has yet to be shown and it is therefore recommended merely to use the word modification to distinguish different crystalline phases of a given protein.