Share-alike is a copyright licensing term, originally used by the Creative Commons project, to describe works or licences that require copies or adaptations of the work to be released under the same or similar licence as the original. Copyleft licences are free content or free software licences with a share-alike condition.
Two currently-supported Creative Commons licences have the ShareAlike condition: Creative Commons Attribution-ShareAlike (a copyleft, free content licence) and Creative Commons Attribution-NonCommercial-ShareAlike (a proprietary licence).
Copyleft or libre share-alike licences are the largest subcategory of share-alike licences. They include both free content licences like Creative Commons Attribution-ShareAlike and free software licences like the GNU General Public License. These licences have been described pejoratively as viral licences, because the inclusion of copyleft material in a larger work typically requires the entire work to be made copyleft. The term reciprocal license has also been used to describe copyleft, but has also been used for non-libre licenses (see, for example, the Microsoft Limited Reciprocal License).
Free content and software licences without the share-alike requirement are described as permissive licences.
As with all six licences in the current Creative Commons suite, CC Attribution-ShareAlike and CC Attribution-NonCommercial-ShareAlike require attribution. According to Creative Commons, the advantage of this license is that future users are not able to add new restrictions to a derivative of your work; their derivatives must be licensed the same way.
The 3.0 and 4.0 version of the ShareAlike licenses include a compatibility clause, allowing Creative Commons to declare other licenses as compatible and therefore derivatives may use these instead of the license of the original work.
Over the years, Creative Commons has issued 5 versions of the BY-SA and BY-NC-SA licenses (1.0, 2.0, 2.5, 3.0 and 4.0).
In June 2009 the Wikipedia community and Wikimedia Foundation board approved the adoption of the Creative Commons Attribution-ShareAlike (CC BY-SA) license as the main content license for Wikipedia and other Wikimedia sites. Creative Commons hailed this decision as a victory for free culture as well as visionary leadership.
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