An aqueduct is a channel built to carry water a long way, often high up on arches so it can cross a river or a valley and keep the water flowing downhill. The famous ones are Roman: a line of stone arches marching across the land, with a watercourse on top. Though actually, an aqueduct does not have to look like a bridge. Some run underground, and some modern ones are just large pipes or concrete channels. The point is the job — bringing water from where it is to where people need it — not the pretty arches, even if those are what most learners picture. It is a slightly formal, textbook word; in casual talk people may just say the water channel or the old Roman bridge. You also meet it in history class more than in daily life.