The Composite Extension of the
X Window System
The X Window System (X11, or simply X) is a windowing system for bitmap displays, common on Unix-like operating systems.
X originated as part of Project Athena at Massachusetts Institute of Technology (MIT) in 1984. The X protocol has been at ...
renders the graphical output of clients "...to an
off-screen buffer. Applications can then take the contents of that buffer and do whatever they like. The off-screen buffer can be automatically merged into the parent window or merged by external programs, called compositing managers."
This enabled the creation of
compositing managers for X, capable of effects like transparency, 3D rotation, and jiggly windows.
The composite extension was added to
X.org in version X11R6.8 in September 2004.
References
{{XWinSys
X Window extensions