Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

thesaurus
     n : a book containing a classified list of synonyms [syn: {synonym
         finder}]
     [also: {thesauri} (pl)]