X Windows is the standard on Unix and Unix based systems for GUI interfaces. It is often referred to as X or X11, and was developed in 1984 at MIT where Bob Scheifler and Jim Gettys set out the early principles of the system. Providing the basic framework for graphical user interfaces on operating systems such as FreeBSD and Linux, version X11 was released in 1987. The X.Org Foundation leads today’s implementation of the project, and other early bitmap display systems included the Xerox Alto, Apple Lisa, and the Mac along with Unix flavors as well.
X gets it’s name from a pre-1983 window system called W, which X follows in the Latin alphabet.