X11
Eleventh version of the X Window System. The X Window System is a network transparent window system which runs on a wide range of computing and graphics machines. X11 has primitives which are useful for the creation of graphical desktops (e.g., Windows, Colors, Displays, Screens). You can almost think of X11 as a 2D graphics library but in practice it does far more than that. X11 is also responsible for delivering a unified stream of events describing the user's interaction with input devices like the keyboard, mouse, touch pads, etc.