Wiktionary:GNU Free Documentation License
The GNU Free Documentation License (GFDL) is a copyleft license for free documentation, designed by the Free Software Foundation (FSF) for the GNU project. The official text of version 1.1 of the license text can be found at: http://www.gnu.org/copyleft/fdl.html
The license is designed for software documentation and other reference and instructional materials. It stipulates that any copy of the material, even if modified, carry the same license. Those copies may be sold but, if produced in quantity, have to be made available in a format which facilitates further editing.