X Window System
A specification for device-independent windowing operations on bitmap display devices, developed initially by MIT's Project Athena and now a de facto standard supported by the X Consortium. X was named after an earlier window system called "W". It is a window system called "X", not a system called "X Windows".
X uses a client-server protocol, the X protocol. The server is the computer or X terminal with the screen, keyboard, mouse and server program and the clients are application programs. Clients may run on the same computer as the server or on a different computer, communicating over Ethernet via TCP/IP protocols. This is confusing because X clients often run on what people usually think of as their server (e.g. a file server) but in X, it is the screen and keyboard etc. which is being "served out" to the applications.