Nomenclature
People in the computer trade commonly shorten the phrase "X Window System" to "X Window", "X11" (for version 11, used since 1987) or simply to "X". The term "X-Windows" (in the manner of "Microsoft Windows") is not officially endorsed, though it has been in common use since early in the history of X and has been used deliberately for provocative effect, for example in the UNIX-HATERS Handbook.
Read more about this topic: X Servers