Source : Free On-Line Dictionary of Computing

working set
     
         The set of all pages (in a {paging} {virtual
        memory} system) used by a {process} during some time interval.
     
        As a result of {locality of reference}, the working set
        frequently consists of a relatively small fraction of a
        process's total virtual memory pages.  While a process's
        entire working set is in {physical memory} the process will
        run without {page faults}.  If the working set is too large
        for available physical memory, the process causes frequent
        {page faults}.
     
        In a {multitasking} environment, information about which pages
        are in each process's working set allows the memory management
        system to improve {CPU} efficiency by {prepaging} (also called
        the {working set model}).
     
        ["Modern Operating Systems", Andrew S. Tanenbaum, pub.
        Prentice Hall, Inc.  1992].
     
        (1997-04-09)