The Collaborative International Dictionary of English v.0.48:

aetiology \ae`ti*ol"o*gy\ ([=e]`t[i^]*[o^]l"[-o]*j[y^]), n. [L.
   aetologia, Gr. a'itiologi`a; a'iti`a cause + lo`gos
   description: cf. F. ['e]tiologie.]
   1. The science, doctrine, or demonstration of causes; esp.,
      the investigation of the causes of any disease; the
      science of the origin and development of things; etiology.
      AS
      [1913 Webster]

   2. The assignment of a cause.
      [1913 Webster]

   3. (Med.) The cause or origin of a disease. Now more commonly
      written etiology.
      [PJC + AS]

The Collaborative International Dictionary of English v.0.48:

Etiology \E`ti*ol"o*gy\, n. [Cf. F. ['e]tiologie.]
   The science of causes. Same as ?tiology.
   [1913 Webster]

WordNet (r) 3.0 (2006):

etiology
    n 1: the cause of a disease [syn: etiology, aetiology]
    2: the philosophical study of causation [syn: etiology,
       aetiology]