Content-Type: text/html
Initially only designed for software distribution, the concept is now also being used for other types of material.
In copyleft, the copyright holder grants an irrevocable license to the recipient of a copy, permitting the redistribution (and even sale) of possibly modified further copies, under the condition that all those copies carry the same license and are made available in a form which facilitates modification.
Examples of copyleft licenses are the GNU General Public License, the [Open Content License]?, and the GNU Free Documentation License. The latter is being used for the contents of this encyclopedia.