The Collaborative International Dictionary of English v.0.48:

Thesaurus \The*sau"rus\, n.; pl. Thesauri. [L. See
   Treasure.]
   A treasury or storehouse; hence, a repository, especially of
   knowledge; -- often applied to a comprehensive work, like a
   dictionary or cyclopedia.
   [1913 Webster]

WordNet (r) 3.0 (2006):

thesaurus
    n 1: a book containing a classified list of synonyms [syn:
         thesaurus, synonym finder]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "thesaurus":
   armory, arsenal, biographical dictionary, cache,
   chemical dictionary, desk dictionary, dialect dictionary,
   dictionary, dictionary of quotations, electronics dictionary,
   etymological dictionary, foreign-language dictionary, gazetteer,
   general dictionary, geological dictionary, gloss, glossary, gradus,
   lexicon, lexis, nomenclator, onomasticon, phrase book, phraseology,
   polyglot dictionary, promptorium, repository, rhyming dictionary,
   science dictionary, slang dictionary, specialized dictionary,
   stock of words, storehouse, synonym dictionary, synonymy,
   terminology, treasure trove, treasury, treasury of words,
   unabridged dictionary, verbiage, vocabulary, word list, wordage,
   wordbook, wordhoard, words