From Computer History Wiki
A system call is the mechanism by which a user's computation, usually running in a process, requests that the operating system's kernel perform some service for it. Since the kernel is usually operating in some specially-privileged way (often in a special mode of the CPU, to which the users do not have access, so that they may be protected from each other), the process must use some special mechanism to transfer, in a carefully controlled way, to the kernel.
This is usually done with a restricted instruction which causes a trap, but other approaches have been used. For instance, Multics used a combination of its single-level memory and protection rings to do system calls with a subroutine call into the kernel.