The GNU Free Documentation License is a copyleft License for Free Documentation

The GNU Free Documentation License (GNU FDL or simply GFDL) is a copyleft license for free documentation, designed by the Free Software Foundation (FSF) for the GNU project. It is the counterpart to the GNU General Public License that gives readers the same rights to copy, redistribute and modify a work and requires all copies and derivatives to be available under the same license. Copies may also be sold commercially, but if produced in larger quantities (greater than 100) then the original document or source code must be made available to the work's recipient.

Mediawiki is a free wiki engine for wikis. Developed under the GNU Free Documentation License, it is used by the Wikimedia Foundation, which hosts Wikipedia, and Wikinews.

The GFDL was released in draft form for feedback in September 1999. After revisions, version 1.1 was issued in March 2000, version 1.2 in November 2002, and version 1.3 in November 2008. The current state of the license is version 1.3

Open source software, software where the source code is publicly available and which is licensed with an open source license.

Examples of free software license / open source licenses include the Apache License, BSD license, GNU General Public License, GNU Lesser General Public License, MIT License, Eclipse Public License and Mozilla Public License.