A viaduct is a bridge structure which extends continuously across multiple crossing points. It must form a series of at least two bridge crossings (over water, over valleys, or over other pathways), with the section(s) between crossings resting on piers.

The elevated route continues between the crossing points on piers. Two or more separate obstacles can therefore be crossed by one continuous structure.

Several spans over a single gap do not alone meet the requirement for at least two bridge crossings. The distinction concerns successive crossings and the elevated connection between them.