Dunes are depositional landforms, comprising accumulations of sediment (sand, silt, or clay), typically deposited by wind (eolian) action. They may be defined according to the type of sediment they comprise (sand dunes, clay dunes), their morphology (linear dunes, barchan dunes, star dunes, parabolic dunes, transverse dunes), their position relative to other geomorphic features in the landscape (barrier dunes, lunette dunes, headland bypass dunes, climbing-falling dunes), the time of their formation (contemporary dunes, paleo- or relict dunes), or even according to their genesis (natural dunes, artificial dunes).

Dunes vary in extent and, typically, reach heights of between 5 m (meters) and 30 m. Megadunes reach 100 m or, in extreme cases, 400 m or more in height. Dunes occur typically, but not exclusively, in hot deserts and ...