The Free On-line Dictionary of Computing (30 December 2018):

page fault

    In a paged virtual memory system, an
   access to a page (block) of memory that is not currently
   mapped to physical memory.  When a page fault occurs the
   operating system either fetches the page in from secondary
   storage (usually disk) if the access was legitimate or
   otherwise reports the access as illegal.

   (1995-11-11)