A pastry, either sweet or savoury, consisting of a crust and filling, sometimes also topping and/or decoration.
It can be any size, open or closed, baked or not, and the crust can be made in countless ways and from countless different ingredients. The filling, of course, also has countless possibilities.
(And, yes, pizza counts as pie.)
PS. In case of a verbal rather than a written question, I might also answer something along the lines of "an irrational number with a infinite number of decimals, usually rounded to 3,14; used a lot in mathematics (e.g. to calculate the area of a circle).
![Cool :cool: :cool:](https://i.imgur.com/8emmqt4.png)