WordNet (r) 3.0 (2006):

system call
    n 1: an instruction that interrupts the program being executed
         and passes control to the supervisor [syn: system call,
         supervisor call instruction]

The Free On-line Dictionary of Computing (30 December 2018):

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)