System Call
In plain terms
A program's request for the operating system to do privileged work.
Going deeper
When code needs to read a file, send network data, or allocate memory, it makes a system call to cross from user mode into the kernel. It's the controlled gateway to hardware and OS services.
Under the hood
Switches from user to kernel mode via a trap; the boundary crossing has overhead, so high-performance I/O batches calls (io_uring, epoll).
Examples
read(), write(), open(), fork(), socket(), mmap().
Think of it like...
Like pressing the service button on a plane: you can't fly the plane yourself, so you ask the crew (the kernel) to do the restricted task.