In physical geography, a dune is a hill of sand built either by wind or water flow. Dunes occur in different forms and sizes, formed by interaction with the flow of air or water. Most kinds of dunes are longer on the windward side where the sand is pushed up the dune and have a shorter "slip face" in the lee of the wind. The valley or trough between dunes is called a slack. A "dune field" is an area covered by extensive sand dunes. Large dune fields are known as ergs.

Some coastal areas have one or more sets of dunes running parallel to the shoreline directly inland from the beach. In most cases the dunes are important in protecting the land against potential ravages by storm waves from the sea. Although the most widely distributed dunes are those associated with coastal regions, the largest complexes of dunes are found inland in dry regions and associated with ancient lake or sea beds.

Dunes also form under the action of water flow (alluvial processes), and on sand or gravel beds of rivers, estuaries and the sea-bed.

The modern word "dune" came into English from French circa 1790. In ancient times, words cognate to "dune" probably had the meaning of a built-up hill or citadel fortification.