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]