The Free On-line Dictionary of Computing (30 December 2018):
synonym ringA list of words with related meanings used to broaden the scope of a word search. As well as returning documents which contain a sought-for word a search using a synonym ring might also return documents which contain words in the same synonym ring as a sought-for word. Synonym rings are produced manually and are usually specific to a certain field, e.g. legal knowhow. (1997-04-09)