On the Internet, in guidebooks and brochures, you'll find two denominations for this big pile of sand: Dune du Pilat and Dune du Pyla. So you will probably want to know which one is right. And the right answer is Dune du Pilat, but only a few people know it.
While in all official documents the historically documented spelling Pilat is used (see for example here), a lot of people use Pyla, especially outside of France. Pilat is derived from the Gascon word "Pilat" which means "heap", "mound".
The seaside resort of Pyla sur Mer, founded in 1920 and being a part of La Teste de Buch extends right north of the dune. This proximity has led to the use of Dune du Pyla and the confusion about the dune's real name.
When you are heading west towards the Atlantic Ocean, you will meet alternately road signs with Dune du Pilat and Dune du Pyla. And to make things even trickier, there are signs showing the way to Dune de Pyla! But don't worry, it's all about the same dune.
Official organisation/Tourist Office:
Syndicat Mixte de la Grande Dune du Pilat