You may have come here looking for information on The X Window System.
The word "Windows" is apparently trademarked by Microsoft, and hence references to X where there is an "s" added to the word "Window" can be argued to be a trademark infringement.
X is commonly called "The X Window System," "X," "X11," "X11R6." Some people prefer to describe it as "X Windows" specifically because it bugs the purists.
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