X Window System
The X Window System is the now-dominant window system used by applications which wish to perform output to a window (either in the form of text, graphics, or images). Unlike earlier window systems, which were part of an operating system, or interacted though subroutine calls, X is based on a protocol which runs over a reliable byte stream. An X session can be run over any data network which provides such streams. (Currently TCP/IP internets are used, but in the past the CHAOS protocol was also used.) The streams connect the application to an X server which has direct access to the display being used.