In pancake sorting, what operation reverses the first k items of a stack?

In pancake sorting, what operation reverses the first k items of a stack? It is a prefix reversal.

A prefix is the beginning portion of a sequence, so reversing the first k pancakes means turning that top block upside down while leaving the rest of the stack unchanged. In a physical version, this is the result of inserting a spatula beneath pancake k and flipping everything above it.

Pancake sorting uses prefix reversals as its only permitted operation. A standard strategy repeatedly brings the largest unsorted pancake to the top, then flips it into its final position. That makes the algorithm resemble selection sort, although its allowed move is a block reversal rather than an ordinary swap.

A suffix reversal would affect the end of a sequence, while a bubble swap exchanges neighboring items. Neither describes the defining move in the pancake problem. The same terminology also appears in combinatorics, permutation theory, computer networks, and models of genome rearrangement.