File:Libya 4608 Idehan Ubari Dunes Luca Galuzzi 2007.jpg
Sand dunes in the Sahara desert region of Fezzan in Libya

In physical geography, a dune is a hill of sand, usually deposited by eolian processes (wind) or sometimes by the flow of water. Dunes occur, for example, in some deserts and beaches. Sand dunes have different forms and sizes based on their interaction with the wind. Sometimes they migrate downwind. A "dune field" is an area covered by many sand dunes.