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

Moby Thesaurus II by Grady Ward, 1.0:

42 Moby Thesaurus words for "lexicology":
   bowwow theory, comparative linguistics, derivation,
   descriptive linguistics, dialectology, dingdong theory, etymology,
   glossematics, glossography, glossology, glottochronology,
   glottology, grammar, graphemics, historical linguistics,
   language study, lexicography, lexicostatistics, lexigraphy,
   linguistic geography, linguistic science, linguistics,
   mathematical linguistics, morphology, morphophonemics,
   onomasiology, onomastics, onomatology, paleography, philology,
   phonetics, phonology, psycholinguistics, semantics, semasiology,
   semiotic, semiotics, significs, sociolinguistics, structuralism,
   syntactics, transformational linguistics