The X Window System, also known as X11 or X, is a popular graphical windowing system that was originally created at the Massachusetts Institute of Technology (MIT) in 1984.

X Windows provides a free cross-platform foundation upon which graphical user interfaces can be created for any computer running a Unix-like operating system. The wide-scale availability of X11 has led to the development of popular open source computer desktop environments such as KDE and GNOME.