thesaurus

Concise Oxford English Dictionary © 2008 Oxford University Press:
thesaurus /θɪˈsɔːrəs/

▶noun (pl. thesauri /-rʌɪ/ or thesauruses)

• 1 a book that lists words in groups of synonyms and related concepts.

• 2 archaic a dictionary or encyclopedia.

– origin C16: via L. from Gk thēsauros ‘storehouse, treasure’.