A hill or ridge of sand accumulated and sorted by wind action. Once a dune is formed, sand will settle on it rather than on bare surfaces. This is because the friction of the sandy surface is enough to slow down the wind, which then sheds some of its load.

Dunes formed in the lee of some obstacle are topographic dunes: lunettes are formed in the lee of a deflation hollow, nebkhas form in the lee of bushes, wind shadow dunes form in the lee of hills and plateaux, although some dunes form windward of such topographic obstacles. Parabolic dunes are hairpin-shaped with the bend pointing downwind and originate around patches of vegetation.

Wind direction is significant. Where the direction is very changeable, star dunes form. The linear seif dunes form when two prevailing winds alternate, either daily or seasonally. When the supply of sand is limited, barchans form with the horns pointing downwind. Barchans may be reshaped into dome dunes by strong winds. Barchanoid dunes are undulating, continuous cross-wind dunes which may grade into long transverse dunes. See also coastal dunes.