The pancake number is the minimum number of flips required to turn a stack of unevenly piled pancakes into a graduated stack where they are distributed from smallest at the top to largest at the bottom.
For a stack in which it doesn't matter which way up the pancakes are, the flipping begins by inserting a spatula and turning from the biggest pancake and continuing in this way until the pile is sorted in order.
For a stack of 5 pancakes, the number of flips required is 5 or fewer. For a stack of 20, the number of flips required is still unknown.
It gets more complicated if one side of the pancake is burnt and the flipping is intended to hide the burnt side from view.