"All text is available under the terms of the GNU Free Documentation License."
If Wikipedia was sourcecode, I suppose it would be released under the GNU general public license.
GNU is a recursive acronym for: GNU's Not Unix.
http://www.gnu.org/ "The Free Software Foundation (FSF) is the principal organizational sponsor of the GNU Project."
Linux is more accurately called GNU/Linux. A lot of the software found within most Linux distributions is part of the GNU project, or at least licensed under the GPL.