To force quit
a running application
is to unceremoniously abort
its operation, giving it no chance to clean up
its work, checkpoint or save its files, or otherwise prepare for its demise. The term "force quit" comes from Mac OS
, under which the operation may be performed with the keystroke Command-Option-Escape.
The Windows equivalent of a Mac OS force-quit is "End Task", available from the Task Manager. The Unix command-line equivalent is kill -9. The X equivalent] is xkill or Ctrl-Alt-Escape, followed by a mouse click in the offending application's window.