I know what you're thinking. You're thinking this is a dinosaur. I'm sorry to say it is not so. The answer to this is more mind bending than you will believe. Try looking it up in the thesaurus. It is, in fact, a synonym for the book of synonyms - the thesaurus itself.
According to Dictionary.com, it is, "a list or collection of specialized terms, as those used in a particular field or subject area". Granted, it isn't quite the same as a thesaurus. It is, however, a much cooler word, and therefore, a much better word. So the next time you need to use the word "thesaurus", prove you have amazing diction to your listeners and use onomasticon in its place.