Confusion often arises when different people use terms to mean different things. In drawing up the following list I am grateful for suggestions and comments from three consultant colleagues who specialise in the development and use of thesauri and other forms of structured vocabulary for information retrieval: Stella Dextre Clarke, Alan Gilchrist and Ron Davies. I have tried as far as possible to maintain consistency with definitions which we drafted for the revised British standards for thesaurus construction (BS 8723:2005 Structured vocabularies for information retrieval) and have taken some account of the definitions in the USA national standard, Z39.19. I do not claim that these definitions are "correct" and that other meanings are "wrong", but I hope that they will be found to be a consistent and well-defined set which will aid communication by encouraging everyone to use the same words with the same meaning.