The Collaborative International Dictionary of English v.0.48:

Lexicology \Lex`i*col"o*gy\ (l[e^]ks`[i^]*k[o^]l"[-o]*j[y^]), n.
   [Gr. lexiko`n lexicon + -logy: cf. F. lexicologie.]
   The science of the derivation and signification of words;
   that branch of learning which treats of the signification and
   application of words.
   [1913 Webster]

WordNet (r) 3.0 (2006):

lexicology
    n 1: the branch of linguistics that studies the lexical
         component of language