The Collaborative International Dictionary of English v.0.48:

Viaduct \Vi`a*duct\, n. [L. via a way + -duct, as in aqueduct:
   cf. F. viaduc. See Via, and Aqueduct.]
   A structure of considerable magnitude, usually with arches or
   supported on trestles, for carrying a road, as a railroad,
   high above the ground or water; a bridge; especially, one for
   crossing a valley or a gorge. Cf. Trestlework.
   [1913 Webster]

WordNet (r) 3.0 (2006):

viaduct
    n 1: bridge consisting of a series of arches supported by piers
         used to carry a road (or railroad) over a valley

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "viaduct":
   Bifrost, bascule bridge, bateau bridge, bridge, cantilever bridge,
   carrefour, catwalk, cloverleaf, crossing, crosspoint, crossroad,
   crosswalk, crossway, drawbridge, floating bridge, flyover,
   footbridge, gangboard, gangplank, gangway, grade crossing,
   intercrossing, intersection, level crossing, lift bridge,
   overbridge, overcrossing, overpass, pontoon bridge, rope bridge,
   rotary, span, stepping-stone, stepstone, suspension bridge,
   swing bridge, toll bridge, traffic circle, undercrossing