The Collaborative International Dictionary of English v.0.48:

Aqueduct \Aq"ue*duct\, n. [F. aqueduc, OF. aqueduct (Cotgr.),
   fr. L. aquaeductus; aquae, gen. of aqua water + ductus a
   leading, ducere to lead. See Aqua, Duke.]
   1. A conductor, conduit, or artificial channel for conveying
      water, especially one for supplying large cities with
      water.
      [1913 Webster]

   Note: The term is also applied to a structure (similar to the
         ancient aqueducts), for conveying a canal over a river
         or hollow; more properly called an aqueduct bridge.
         [1913 Webster]

   2. (Anat.) A canal or passage; as, the aqueduct of Sylvius, a
      channel connecting the third and fourth ventricles of the
      brain.
      [1913 Webster]

WordNet (r) 3.0 (2006):

aqueduct
    n 1: a conduit that resembles a bridge but carries water over a
         valley