Source : Free On-Line Dictionary of Computing

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)