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.
The license explicitly separates the "Document" from "Secondary Sections", which may not be integrated with the Document, but exist as front-matter materials or appendices. Secondary sections can contain information regarding the author's or publisher's relationship to the subject matter, but not any subject matter itself. While the Document itself is wholly editable, and is essentially covered by a license equivalent to the GNU General Public License, some of the secondary sections have various restrictions designed primarily to deal with proper attribution to previous authors.
Specifically, the authors of prior versions have to be acknowledged and certain "invariant sections" specified by the original author and dealing with his or her relationship to the subject matter may not be changed. If the material is modified, its title has to be changed (unless the prior authors give permission to retain the title). The license also has provisions for the handling of front-cover and back-cover texts of books, as well as for "History", "Acknowledgements", "Dedications" and "Endorsements" sections.
Using the GFDL
For a document to be covered by the GFDL, one must include a specific copyright and license notice.
Wiktionary and GFDL
All Wiktionary articles are licensed to the public under the GNU Free Documentation License. See Wiktionary:Copyrights for the details. The local copy of the license, as required by the terms of the GFDL, is at Wiktionary:Text of the GNU Free Documentation License.