The GNU Free Documentation License (GNU FDL or simply GFDL) is a copyleft license for free content, designed by the Free Software Foundation (FSF) for the GNU project. It is the open content counterpart to the GNU GPL. The current state of the license is version 1.2, the official text of which can be found at http://www.gnu.org/copyleft/fdl.html.

The license was designed for manuals, textbooks, and other reference and instructional materials. However, it can be used for any text-based work, regardless of subject matter. 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..