(Or "Xemacs")
A text editor for the X Window System, based on an early version of Emacs version 19 from the Free Software Foundation, produced by a collaboration of Lucid Inc. with SunPro (a division of Sun Microsystems Inc.) and the University of Illinois.
Lucid chose to build part of Energize, their C/C++ development environment on top of GNU Emacs. Though their product is commercial, the work on GNU Emacs is free software, and is useful without having to purchase the product. They needed a version of Emacs with mouse-sensitive regions, multiple fonts, the ability to mark sections of a buffer as read-only, the ability to detect which parts of a buffer has been modified, and many other features.
The existing version of Epoch was not sufficient; it did not allow arbitrary pixmaps and icons in buffers, "undo" did not restore changes to regions, regions did not overlap and merge their attributes. Lucid spent some time in 1990 working on Epoch but later decided that their efforts would be better spent improving Emacs 19 instead.
Lucid did not have time to get their changes accepted by the FSF so they released Lucid Emacs as a forked branch of Emacs. Roughly a year after Lucid Emacs 19.0 was released, a beta version of the FSF branch of Emacs 19 was released. The FSF version is better in some areas, and worse in others. Lucid continue to develop and support Lucid Emacs, merging in bug fixes and new features from the FSF branch as appropriate. They feel that Lucid Emacs has a cleaner and more extensible substrate, and that it would be easier to merge the FSF changes into their version than the other way around.
Epoch and Lucid Emacs will soon be one and the same thing. Work is being done on a compatibility package which will allow Epoch 4 code to run in Lemacs with little or no change. (As of 19.8, Lucid Emacs is running a descendant of the Epoch redisplay engine.)