The common name for MicrosoftWindows.
Note: MicrosoftCorporation does NOT own this name! MicrosoftCorporation tried to sue Lindows.com (now Linspire) over dilution of their trademark, but the judges said that Windows was a common term used for any windowing system and there was plenty of prior art before they came along.
The X Windowing System is NOT EVER to be called "X Windows."
62 pages link to Windows: