What is an explanation of the term 'page fault'?

A page fault in any operating system will occur when a reference to memory indicates that the actual information has been paged out to disk, and is not available in memory at the present time.

When the page fault occurs, the system has to swap that page back into memory before it can reference the memory location. This is how virtual memory was implemented.

Page faults should be transparent to the program; it doesn't know or care if the memory was resident or not, since the net effect is to make that memory available to the program, whether it was paged out or not.