System call syscall
Function call from a user-space process into the kernel — to read a file, send a network packet, create a process. The only door between user space and kernel.
x86_64 uses the syscall instruction (formerly int 0x80 or sysenter). Linux has ~400 syscalls; macOS/BSD have their own numbers. Much more expensive than a regular function (~100–1000ns vs 1–5ns) due to context switching. Optimizations: io_uring (Linux 5.1+, async syscall queue), vDSO (some calls like gettimeofday run without trapping). Tools: strace traces every syscall a process makes.