From The Collaborative International Dictionary of English v.0.48 [gcide]: 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]
From WordNet (r) 3.0 (2006) [wn]:
thesaurus
n 1: a book containing a classified list of synonyms [syn:
thesaurus, synonym finder]