Page fault
The trap raised when a process touches a memory address not currently in physical memory — not an error in the usual sense.
Despite the name, most page faults are entirely normal and a precondition for virtual memory to work at all. There are two kinds. A minor fault is resolved without disk: the page is already in memory but was not mapped into the process, or is to be allocated now because it was asked for. A major fault requires a read from disk, which takes orders of magnitude longer.
That is why the number of major faults is a better health indicator than memory usage itself. If it climbs sharply the system is thrashing, spending all its time moving pages rather than computing. A genuine error, such as dereferencing a null pointer, also raises a page fault, but one the kernel cannot satisfy — and then it becomes a segmentation fault.