A lexicon is a list of words that belong to a particular language. Sometimes, lexicon is used as another word for thesaurus (see below) A dictionary is a list of words and phrases that are (or were) in common usage, together with their definitions - so a dictionary is different from a lexicon because a lexicon is a simple list and doesn't define the words. A thesaurus is a dictionary of ...
Consequently, lexicon is often, but not invariably, the preferred designation of a dictionary that interprets words of one language in terms of another,; as, a Latin-English lexicon; an English-French lexicon.
6 You can also try lexicon. Noun The vocabulary of a person, language, or branch of knowledge. A dictionary, esp. of Greek, Hebrew, Syriac, or Arabic: "a Greek–Latin lexicon". Synonyms dictionary - vocabulary - wordbook - thesaurus - glossary
34 I would use the word lexicon. noun the vocabulary of a person, language, or branch of knowledge. Quoted from Oxford Dictionaries Online.
EDIT: I wrote that lexicon is a synonym of vocabulary, and it is, but Rhodri made me notice that it's also a synonym of dictionary. So I investigated a bit and, as I was suspecting, it has a restricted use, which is still right though... From the OED: A word-book or dictionary; chiefly applied to a dictionary of Greek, Hebrew, Syriac, or Arabic.
A lexicon is just a catalog or dictionary of terms. Terminology is the set of specialized terms in my field of study. These items are clearly understood by others in my field of study. Jargon is a set of terms used by people in other fields of study. These terms are confusing, ambiguous and frustrating.