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]