Flaky pastry consisting of multiple layers of pastry dough and butter or fat. As the dough cooks in the oven it rises, although there is no leavening, because of the way it is prepared. The dough is rolled into thin slabs that are stacked with butter added in between each layer. In the oven the butter melts and boils and as the water evaporates into steam it pushes the layers of dough up. At the same time the flour is cooking and hardening around the air bubbles. 

This is what the French call mille-feuille. Puff pastry dough is a relative of the Middle Eastern fyllo (fillo or filo) dough.  

The dough is used for sweet pastries with various fruits and cream fillings, such as strudel, apple turnover, French King Cake and also for savory items such as beef Wellington and turnovers filled with veggies or meat.