The clear and unambiguous definition of basic terms in a field of science has special significance. Subsequent nomenclature must refer back to these basic terms, and the utmost care is, therefore, required in preparing and revising them. In 1974, the Commission published a document entitled ''Basic Definitions of Terms Relating to Polymers (1974)'', that appeared in Pure Appl. Chem. 1974, 40, 479-491. These definitions have been the foundation for the Commission's work and, building on this basis, the Commission has developed a number of further nomenclature documents. The vast majority of terms in the basic document serve their purpose well. Progress in polymer science and the need for new definitions, the proper wording of which would have been incompatible with some of the basic terms, made it increasingly apparent that some change was needed in the basic terms. Approximately twenty years since the publication of the first document, the Commission now issues a revised and enlarged set of basic terms. The new glossary of terms has been formulated by the Commission with the additional input and aid of several well-known researchers and journal editors.