General:
* Vocabulary
* Word list
* Glossary
* Dictionary
* Terminology
* Jargon
* Idiom
More specific:
* Technical vocabulary
* Slang
* Argot
* Specialized vocabulary
* Register
* Language
* Word stock
* Verbal repertoire
The best synonym will depend on the specific meaning of "lexicon" in the context.
For example, if you're talking about the vocabulary of a particular profession, you might use "technical vocabulary" or "specialized vocabulary." If you're talking about the slang used by a particular group of people, you might use "argot" or "jargon."