page fault

The interrupt that fires when a program touches memory that isn't currently mapped in.

When a program accesses a virtual address the MMU can't translate — because the page isn't loaded, doesn't exist, or is forbidden — the hardware raises a page fault and hands control to the OS. The OS then either fixes it (loading the page, growing the stack) or, for a genuinely invalid access, kills the program with a segfault. It's the pivot point of virtual-memory handling.