The RK&M glossary defines important concepts and terms that are commonly used. It is under continuous development by the project team and represents a peer-reviewed set of terms. It should be pointed out that some terms may be defined differently from other glossaries in science and technology. The RK&M glossary, however, fits the purpose of achieving a better understanding and more efficient communication of RK&M issues. The aim of the RK&M glossary is to compile a dedicated, internally consistent, and unambiguous set of definitions. Each definition should not be seen in isolation, but as complementary to the definition of other terms in the glossary. This document represents the fourth revision of the glossary.