The Free On-line Dictionary of Computing (30 December 2018):

synonym ring

    A 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)