Define stack as an Abstract Data Type. Define the operations without concern of implementation, memory usage or algorithms to be used. For example, a phone, makes calls, receives calls. It does not matter type, company, for smartphones we not are concerned with the operating systems. For a stack you be more specific: Do we add to it? What do we call those methods? Do we remove from it? What do we call those methods? Is there a specific manner in which we add? Can we get information about the structure? size? certain elements?