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