A "stack" is a data structure used in computer science that follows a 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 tasks such as managing function calls, undo mechanisms in applications, and parsing expressions. In practical terms, a stack can be visualized like a stack of plates. You can only add or remove the top plate, making it easy to keep track of the most recent items. Common operations associated with stacks include push (adding an item) and pop (removing the top item).