the·sau·rus 
(thĭ-sôrəs)
(thĭ-sôrəs)n. pl. the·sau·rus·es or the·sau·ri (-sôrī′)
1. A book of synonyms, often including related and contrasting words and antonyms.
2. A book of selected words or concepts, such as a specialized vocabulary of a particular field, as of medicine or music.
[Latin thēsaurus, treasury, from Greek thēsauros.]