Strictly, an aqueduct is a watercourse constructed to convey water, which can include ditch, bridge, tunnel, pipe etc. In that sense, the whole of the Llangollen Canal might be considered an aqueduct as to conveys water from the Horseshoe Falls to the Shopshire Union and Hurleston Reservoir. When I visited the Pont Du Gard aqueduct in France, it was explained that the Romans considered the entire watercourse an aqueduct, not just the bridge.
Our definition of aqueduct tends to mean any bridge carrying a watercourse regardless of the number of spans. It would perhaps make sense to define multi-span aqueducts as a specific type of viaduct but it seems the word viaduct was only derived from aqueduct around the time of the building of the railways.