A stack, or pile, is a collection of items stacked on top of one another. This arrangement can be seen in various contexts, such as a stack of books, a pile of leaves, or a stack of plates. The items in a stack are typically organized in a vertical manner, making efficient use of space. In computer science, a stack is a data structure that follows the Last In, First Out (LIFO) principle. This means that the last item added to the stack is the first one to be removed. Stacks are commonly used in programming for managing function calls and storing temporary data.