A stack is an abstract type having two operations: push(element) and pop(). The implementation can cheat till it's transparent to the user. To quote CLRS, The

A stack is an abstract type having two operations: push(element) and pop(). The implementation can cheat till it's transparent to the user. To quote CLRS, The INSERT operation on a stack is often called PUSH, and the DELETE operation, which does not take an element argument, is often called POP. These names are allusions to physical stacks, such as the spring-loaded stacks of plates used in cafeterias. The order in which plates are popped from the stack is the reverse of the order in which they were pushed onto the stack, since only the top plate is accessible.