The Collaborative International Dictionary of English v.0.48:

Dune \Dune\ (d[=u]n), 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.]
   [1913 Webster]

         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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

dune
    n 1: a ridge of sand created by the wind; found in deserts or
         near lakes and oceans [syn: dune, sand dune]