A lexicon is the vocabulary of a language or subject. "No-hitter," "go-ahead run," and "Baltimore chop" are part of the baseball lexicon.
Lexicons are really dictionaries, though a lexicon usually covers an ancient language or the special vocabulary of a particular author or field of study. In linguistics, the lexicon is the total stock of words and word elements that carry meaning. Lexicon is from Greek lexikon (biblion) meaning "word(book)," ultimately going back to legein "to speak."