The Collaborative International Dictionary of English v.0.48:

Lexicon \Lex"i*con\ (l[e^]ks"[i^]*k[o^]n), n. [Gr. lexiko`n (sc.
   bibli`on), neut. of lexiko`s of or belonging to words, fr.
   le`xis a speaking, speech, a way of speaking, a single word
   or phrase, fr. le`gein to say, to speak. See Legend.]
   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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

lexicon
    n 1: a language user's knowledge of words [syn: vocabulary,
         lexicon, mental lexicon]
    2: a reference book containing an alphabetical list of words
       with information about them [syn: dictionary, lexicon]

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "lexicon":
   biographical dictionary, cant, chemical dictionary,
   desk dictionary, dialect dictionary, dictionary,
   dictionary of quotations, electronics dictionary,
   etymological dictionary, foreign-language dictionary, gazetteer,
   general dictionary, geological dictionary, gloss, glossary, gradus,
   jargon, language, lexis, nomenclator, onomasticon, palaver,
   phrase book, phraseology, polyglot dictionary, promptorium,
   rhyming dictionary, science dictionary, slang dictionary,
   specialized dictionary, stock of words, synonym dictionary,
   synonymy, terminology, thesaurus, treasury of words,
   unabridged dictionary, verbiage, vocabulary, word list, wordage,
   wordbook, wordhoard, words