ShareAlike

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).

The term has also been used outside copyright law to refer to a similar plan for patent licensing.

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.

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Wikipedia