lexicon \LEK-suh-kahn\, noun:
A vocabulary, or book containing an alphabetical arrangement of the words in a language or of a considerable number of them, with the definition of each; a dictionary; especially, a dictionary of the Greek, Hebrew, or Latin language.
"He thought it right in a lexicon of our language to collect many words which had fallen into disuse." --Boswell, Life of Johnson
"Hundreds of thousands of ethnic Albanians fled their homes during the fighting and became, in the lexicon of relief workers, IDPs, or internally displaced persons." --"Casualties of War: In the Hills, Scraping to Survive; Some Ethnic Albanians Have No Reason to Go Home," Washington Post Tuesday, June 15, 1999
"Upon enquiry about a Hebrew word, I found he had no Lexicon." --S. Sewall Diary, January 30, 1879
From lexis, the Greek word for "speaking, speech; a way of speaking; diction, style," from legein, to speak; to tell; to recount.
Lexicon is frequently used to signify the vocabulary of a particular discipline, branch of knowledge, or sphere of activity (as in the second example above: "...in the lexicon of relief workers..."); or of a region, a particular speaker, etc.