A pie is a baked dish which is usually made of a pastry dough casing that contains a filling of various sweet or savoury ingredients. Sweet pies may be filled with fruit (as in an apple pie), nuts (pecan pie), brown sugar (sugar pie), sweetened vegetables (rhubarb pie), or with thicker fillings based on eggs and dairy (as in custard pie and cream pie). Savoury pies may be filled with meat (as in a steak pie or a Jamaican patty), eggs and cheese (quiche) or a mixture of meat and vegetables (pot pie).

Pies are defined by their crusts. A filled pie (also single-crust or bottom-crust), has pastry lining the baking dish, and the filling is placed on top of the pastry but left open. A top-crust pie has the filling in the bottom of the dish and is covered with a pastry or other covering before baking. A two-crust pie has the filling completely enclosed in the pastry shell. Shortcrust pastry is a typical kind of pastry used for pie crusts, but many things can be used, including baking powder biscuits, mashed potatoes, and crumbs.