Copyleft

Copyleft is (obviously and as a typical example of hacker humor) a pun on the word copyright. It is a legal instrument based on copyright that makes sure the copylefted material cannot be unilaterally removed from the commons, available for the general public to the extent that they can persuade someone to provide a copy and thus the material cannot be appropriated by private interests. This means no additional restrictions can be placed on the material. Not following these rules constitutes a copyright infringement.

One could say it's like public domain but with an insurance. However, unlike public domain, copylefted material remains under copyright and derivative versions can only be made if they are similarly licensed. Stated differently, a game can be free without being under copyleft. Copyleft is simply a mechanism which ensures that if the game is free, modified versions will also be.

Copyleft was designed and developed by Richard Stallman and the Free Software Foundation in the 1980s.

The first popular license to embed copyleft was the GNU GPL 1.0 in 1989. A similar construct can be seen in the Creative Commons Share Alike licensing provision.