Here's why it's likely not a standard term:
* Dunes form in specific ways: Dunes are formed by the wind carrying and depositing sand. They typically have a distinct shape, with a windward slope (the side facing the wind) and a leeward slope (the side facing away from the wind).
* "Back" is ambiguous: There's no established meaning of "back" in relation to dunes. It could be interpreted in several ways, none of which clearly define a unique dune type.
What might be meant by "backdune"?
* A dune behind another dune: It's possible someone is referring to a dune that forms behind a primary dune, perhaps in a sheltered area.
* A dune on the backside of a feature: It's possible someone is referring to a dune on the leeward side of a hill or other landform.
* A local term: There might be a regional or local use of the term "backdune" that isn't widely known.
To get a clearer answer, you'd need more context.
* Where did you hear the term "backdune"?
* What was the context of the discussion?
Providing more information might help us understand what the speaker meant by "backdune."
