A DataStructure organized like a stack of books - you can add things on top, ie push them on the Stack, and remove them from the top, ie pop them off the Stack. This also means that you get things out of a Stack in reverse insertion order, a principle referred to as LIFO.
The term was coined by EdsgerWybeDijkstra.
Some Stacks, such as in Forth, allow additional advanced operations like rotation - an operation you'd perform on a stack of books by lifting a smaller stack from the top of the complete one, putting them down on the floor, then putting the rest of the original stack on top of it. This way, the item from X elements deep on the original stack comes to the top, while the items above it move to the bottom of the stack.