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

Moby Thesaurus II by Grady Ward, 1.0:

55 Moby Thesaurus words for "aqueduct":
   arroyo, bed, canal, canalization, channel, conduit, course,
   creek bed, crimp, culvert, cut, dike, ditch, donga, dry bed, duct,
   entrenchment, flume, fosse, goffer, gulch, gully, gullyhole,
   gutter, ha-ha, headrace, irrigation ditch, kennel, moat, nullah,
   pleat, race, river bed, riverway, runnel, sluice, spillbox,
   spillway, stream bed, streamway, sunk fence, swash, swash channel,
   tailrace, trench, trough, wadi, water carrier, water channel,
   water furrow, water gap, water gate, watercourse, waterway,
   waterworks