Source : Free On-Line Dictionary of Computing

stack pointer
     
         (SP) A {register} or variable
        pointing to the top of a {stack}.  If the stack is a "full"
        stack the SP points to the most recently {push}ed item, else
        if it is an "empty" stack, the PS points to the first empty
        location, where the next item will be pushed.
     
        (1995-03-21)