Any collection of sand grains moulded into a mound or ridge by the wind under the action of gravity is referred to as a sand dune.
Sand dunes are similar to other formations that emerge when a fluid travels over a loose substrate, such as subaqueous “dunes” on riverbeds and tidal estuaries, and sand waves beneath shallow oceans.
Dunes can be seen in deserts, on beaches, and even on degraded and abandoned farm fields in semiarid locations like northwest India and sections of the southwestern United States, where loose sand is windblown.
Final answer:
Hence, sand dunes are collection of sand grains moulded into a mound or ridge.