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

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)