Dune \Dune\, n. [The same word as down: cf. D. duin. See Down a bank of sand.]

A low hill of drifting sand usually formed on the coats, but often carried far inland by the prevailing winds. [Written also dun.]

Three great rivers, the Rhine, the Meuse, and the Scheldt, had deposited their slime for ages among the dunes or sand banks heaved up by the ocean around their mouths. --Motley.

Source: Webster's Revised Unabridged Dictionary (1913)