ISO 12749-5:2018 encompasses the collection of terms, definitions, notes and examples corresponding to nuclear reactors, excluding quantitative data. It provides the minimum essential information for each nuclear reactor concept represented by a single term. Full understanding of concepts requires background knowledge of the nuclear field. It is intended to facilitate communication and promote common understanding. The scope of ISO 12749-5:2018 covers the whole field of nuclear reactors at a broad surface level.