Source : Webster's Revised Unabridged Dictionary (1913)

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}.

Source : WordNet®

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