The term "pie shell" refers to the base or crust of a pie that holds the filling. There are several synonyms for the term, including pie crust, pastry shell, pie base, tart shell, and tart crust. A pie crust is typically made from flour, butter, and water or milk, and is rolled out and placed in a pie dish before being filled. A pastry shell is similar, but may be a bit thicker and richer, often made from a mixture of flour, butter, sugar, and egg. A tart shell or crust is usually smaller, thinner, and more delicate than a pie crust, sometimes made with ground nuts. Regardless of the term used, a good pie shell is an essential part of any delicious pie.