The X Window System (commonly known as X11, based on its current major version being 11, or shorted to simply X) is a computer software system and network protocol that provides a basis for graphical user interfaces (GUIs) and rich input device capability for networked computers. It creates a hardware abstraction layer where software is written to use a generalized set of commands, allowing for device independence and reuse of programs on any computer that implements X.
X originated at the Massachusetts Institute of Technology (MIT) in 1984. The protocol version has been X11 since September 1987. The X.Org Foundation leads the X project, with the current reference implementation, X.Org Server, available as free and open source software under the MIT License and similar permissive licenses.