(Originally "Lucid Emacs") A text editor for the X Window System, based on GNU Emacs version 19, produced by a collaboration of Lucid, Inc., 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. Lucid continued to develop and support Lucid Emacs, merging in bug fixes and new features from the FSF branch as appropriate.