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]

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "etiology":
   accounting for, answerability, antecedents, application,
   arrogation, ascription, assignation, assignment, attachment,
   attribution, base, basis, blame, call, causation, cause,
   cause and effect, charge, connection with, credit, derivation from,
   determinant, determinative, element, factor, ground, grounds,
   honor, imputation, occasion, palaetiology, placement, principle,
   reference to, responsibility, saddling, stimulus