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)