Source : Free On-Line Dictionary of Computing

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)