"The main aim in producing this reference work is to try to standardise the terminology in the field of astronomy for the purposes of aiding unambiguous library cataloguing and more precise recall of data from computer databases. The thesaurus is intended for use by astronomy librarians and scientists. Its compilation was originally requested during a meeting of Commission 5 (Documentation) of the International Astronomical Union at the New Delhi General Assembly of the IAU in 1984. Vocabulary control has become essential in the use of online databases, particularly where those responsible for data input and for searching databases are nonspecialists. Consistency, particularly in the input to computerised databases, is imperative if there is to be high precision recall in information retrieval. Conversely, for those accessing databases (the output process), the selection of relevant terms or those most specific to the information request, will result in higher precision recall. The thesaurus is also designed to give the librarian, cataloguer or indexer, as well as the astronomer, a context for astronomical terms with which they may not be familiar. It is envisaged that the thesaurus will be used for the following main purposes: 1. Assigning keyterms to scientific papers; this is usually best done by the authors themselves, rather than by non-specialists. If the thesaurus is to serve as originally intended by Commission 5 of the IAU - to standardise the astronomical terminology -, then authors should use only the upper case terms in the thesaurus in their keyterm lists. 2. Extraction of references from databases, when the use of the same list of terms as is used by the authors can be expected to result in high precision recall of relevant data. 3. Assisting in the updating of library classification schemes. 4. It may be included in 'intelligent' databases, to help the user select alternatives or extensions to a searcher's initial list, as in proximity searching. Other uses suggested to us have been as a source of terms for exclusion as passwords for observatory computers and as a supplementary dictionary of words used by a spelling checker for word processors."