Source : WordNet®

system call
     n : an instruction that interrupts the program being executed
         and passes control to the supervisor [syn: {supervisor
         call instruction}]

Source : Free On-Line Dictionary of Computing

system call
     
         The mechanism used by an
        {application program} to request service from the {operating
        system}.  System calls often use a special {machine code}
        instruction which causes the processor to change mode (e.g. to
        "supervisor mode" or "{protected mode}").  This allows the OS
        to perform restricted actions such as accessing hardware
        devices or the {memory management unit}.
     
        (1995-03-01)