X Window System
System software
The X Window System is a windowing system for bitmap displays, common on Unix-like operating systems.
X provides the basic framework for a GUI environment: drawing and moving windows on the display device and interacting with a mouse and keyboard.... Wikipedia
Initial release date: June 1984
Port number: 6000
Developer(s): X.Org Foundation
License: MIT License
Original author(s): Project Athena
Stable release: X11R7.7 / 6 June 2012