“A lexicon is a collection, or stock, of words. This stock may be characteristic of an individual, a profession, a philosophy, or a ‘style’ (such as, say, a sermon), or it may be as unrestricted as an unabridged dictionary. A lexicon is usually to be distinguished from a glossary in being less focused and less restrictive. It could be argued, for instance, that this glossary need not, even ought not, contain the word lexicon in that, as is the case with many or the words discussed here, it is not a word with a special meaning within the analysis of rhetoric. I would respond that insofar as it is a word used to describe the range of vocabulary from which a given discipline or individual chooses, it is a word that can be useful to the rhetor. See ARGOT.”
Excerpted from: Trail, George Y. Rhetorical Terms and Concepts: A Contemporary Glossary. New York: Harcourt Brace, 2000.