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
A viaduct is a long, high bridge that carries a road or a railway across a valley. a long high bridge, especially one with arches, that crosses a valley and has a road or railway on it (via + -duct (as in aqueduct))