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 callThe 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)