"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. Discussions about some of the definitions are continuing, and they may change. Some of the definitions, notes and examples are my personal suggestions, and my colleagues may not agree with them."