Merriam-Webster Collegiate Dictionary - thesaurus

 noun  (plural thesauri or ~es)  Etymology: New Latin, from Latin, treasure, collection, from Greek thesauros  Date: circa 1823  1. treasury, storehouse  2.  a. a book of words or of information about a particular field or set of concepts; especially a book of words and their synonyms  b. a list of subject headings or descriptors usually with a cross-reference system for use in the organization of a collection of documents for reference and retrieval  • thesaural adjective