The X window system (commonly X Window System or X11, based on its current major version being 11) 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 current protocol version, X11, appeared in 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.

X is an architecture-independent system for remote graphical user interfaces and rich input device capabilities which allows many people to share the processing power of a time-sharing computer and to collaborate with each other through client applications running on remote computers. Each person using a networked terminal has the ability to interact with the display with any type of user input device. Due to the ubiquity of support for X software on Unix, Linux and Mac OS X, X is commonly used to run client applications on personal computers even when there is no need for time-sharing.

A windowing system (or window system) is a component of a graphical user interface (GUI), and more specifically of a desktop environment, which supports the implementation of window managers, and provides basic support for graphics hardware, pointing devices such as mice, and keyboards. The mouse cursor is also generally drawn by the windowing system.

The term windowing system is sometimes used to refer to other elements of a graphical interface such as those belonging to window managers or even applications. While on some operating systems the distinction between applications, window managers, and their supporting technologies are blurred, strictly speaking, a windowing system does not include windows themselves.

From a programmer's point of view, a windowing system implements graphical primitives such as rendering fonts or drawing a line on the screen, effectively providing an abstraction of the graphics hardware from higher level elements of the graphical interface like window managers.