The Collaborative International Dictionary of English v.0.48:

Morphology \Mor*phol"o*gy\, n. [Gr. morfh` form + -logy: cf. F.
   morphologie.]
   1. (Biol.) That branch of biology which deals with the
      structure of animals and plants, treating of the forms of
      organs and describing their varieties, homologies, and
      metamorphoses. See Tectology, and Promorphology.
      [1913 Webster]

   2. (Biol.) The form and structure of an organism.
      [PJC]

   3. (Linguistics) The branch of linguistics which studies the
      patterns by which words are formed from other words,
      including inflection, compounding, and derivation.
      [PJC]

   4. Specifically: The study of the patterns of inflection of
      words or word classes in any given language; the study of
      the patterns in which morphemes combine to form words, and
      the rules for combination; morphemics; as, the morphology
      of Spanish verbs; also, the inflection patterns
      themselves.
      [PJC]

WordNet (r) 3.0 (2006):

morphology
    n 1: the branch of biology that deals with the structure of
         animals and plants
    2: studies of the rules for forming admissible words
    3: the admissible arrangement of sounds in words [syn:
       morphology, sound structure, syllable structure, word
       structure]
    4: the branch of geology that studies the characteristics and
       configuration and evolution of rocks and land forms [syn:
       morphology, geomorphology]