What is a backdune?

"The term backdunes refers to stabilized dunes that are separated from the shoreline by other dunes offering some measure of protection from the wind. Because these dunes have little to no wind erosion and sand accumulation, trees establish themselves, forests develop and soils form.Backdunes are not a distinct type of dune based on shape or method of formation. Their topography shows them to be stabilized parabolic dunes, foredunes and dune ridges. Their forest cover shows that they have not been active for many years."

http://www.calvin.edu/academic/geology/coastaldunes/dunes/backdunes.htm