The phrase most recent versions of Microsoft Windows uses to identify a program that has, as far as it can tell, hung. This is indicated by a white or faded-out window, the hourglass or, in Vista, the swirlie, and the text (Not Responding) in the title bar.
Windows regularly sends window messages to running applications that have a presence on the desktop and expects replies. In old versions of Windows, there is exactly one thread that does this, and any program not responding in a timely manner would simply freeze the entire system until, and if, it got around to releasing it. This causes some obvious stability problems.
Newer versions of Windows are more practical. Each program gets its own message pump, and Windows itself will never hang waiting for a reply. It is, however, paying attention. If a program fails to service the message pump for a consecutive five seconds, it is deemed to be Not Responding and subject to immediate termination.
Windows will try to force a program this thoroughly hung to exit upon clicking the X, exactly as if it was on the wrong end of the End Task button in Task Manager, instead of sending a (futile) WMCLOSE message as it usually does. It doesn't always work.