1. A network windowing environment commony used on UNIX-based workstations. X Windows is a device-independent, application program interface (API) that can run under operatings systems ranging from disk operating systems to a mainframe operating system. It's used most frequently on UNIX machines. Unlike Microsoft Windows 95 and other PC-based windowing environments, X Windows is designed for use on a minicomputer-based network. From QUECID

2. X Windows forms the basis for most GUIs on UNIX. It is based upon a network protocol such that a program can run on one computer but be displayed on another. Conceptually, it is a graphical version of Telnet. Key point: X Windows goes in the "wrong" direction. When you log into an X Windows host, the host opens a connection back to the display. As a consequence, it is very useful as a back-channel. In particular, the program xterm provides a raw command-prompt from which a hacker can interact with just as if they had telnetted to the machine. From Hacking-Lexicon