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].