the GNU version of the Collaborative International Dictionary of English
n. A dictionary of synonyms.
The Century Dictionary and Cyclopedia
n. A dictionary of synonymous words.
Word Usage
"A book of synonyms, often including related and contrasting words and antonyms. synonym = "synonymicon" (or you might use a 'brand name', e.g., "look it up in your Roget's" just as you might use "Webster's" as a general term for dictionary)."