1. The word lexicon refers to a word-book or dictionary mainly of Greek, Hebrew, Syriac, or Arabic. The restricted use is due to the fact that until recently dictionaries of these particular languages were usually in Latin. Lexicon, not dictionarius has been the word generally used.
2. The vocabulary or word-stock of a sphere of activity, a region or a particular speaker.
3. In Linguistics. The complete set of meaningful units in a language.