Acronym for Common Desktop Environment
The "Standard" GUI interface for Unix, based on the Motif toolkit. Since the Motif toolkit was proprietary1?, the OpenSource community didn't really embrace it much, and it got steamrolled by GNOME and KDE. I believe SunMicrosystems (one of the major supporters of CDE) have decided in the latest releases of Solaris to bundle GNOME as well as CDE and give users the option of which to use.
1?: Although there was "lesstif" which was an open source replacement, it was much later.
One page links to CDE:
