pithivier

Can some one tell me what is the difference between a pie and a pithivier? What is a pithivier? I've only ever eaten pies I think.

Another recipe that caught my attention this morning is this one. http://www.greatbritishchefs.com/recipes/vegan-mushroom-chestnut-pithivier-recipe It is mushroom and sweet chestnut time here (well almost on the sweet chestnuts)... and this recipe calls for both of them :D You could easily add...