A causeway is a road or railway route across a broad body of water or wetland raised up on an embankment. Some causeways may only be usable at low tide and the distinction between causeways and viaduct can become blurred when flood-relief culverts are incorporated in the structure; a causeway is however primarily supported on earth or stone, whereas a bridge or viaduct is mainly supported by free-standing columns and/or arches.

When first used, the word appeared in a form such as “causey way” making clear its derivation from the earlier form “causey”. This word seems to have come from the same source by two different routes. It derives ultimately, from the Latin for heel, calx, and most likely comes from the trampling technique to consolidate earthworks. Anciently, the construction was trodden down, one layer at a time, often by slaves or flocks of sheep. Today, this work is done by machines. The same technique would have been used for road embankments, raised river banks, sea banks and fortification earthworks. (The layers, though not the trampling action, can be seen in the Bayeux Tapestry: Building Hastings Castle.)