From 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]
From 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)