1. In programming, a special type of data structure in which items are removed in the reverse order from that in which they are added, so the most recently added item is the first one removed. This is also called last-in, first-out (LIFO). Adding an item to a stack is called pushing. Removing an item from a stack is called popping. 2. In networking, short for protocol stack. 3. In Apple Computer's HyperCard software system, a stack is a collection of cards. 4. In programming, the memory area reserved for a predetermined amount of memory usage, as opposed to the heap.