The X Window System (also called X11 or just X) is a basic framework for building graphical user interfaces on OpenVMS and Unix-like operating systems, such as Linux and BSD.
X consists of a base toolkit and protocol stack that allow any type of user input device to interact with bitmap displays. The protocol stack consists of protocols that operate on OSI layer 7 (also called the application layer), which supports communications between end users and applications and is involved in the presentation of data for user-facing applications.