The specific definition used by Creative Commons is that "If you alter, transform, or build upon this work, you may distribute the resulting work only under the same, similar or a compatible license." However, generic variations of share-alike licenses define free software and open content. The term copyleft has been used since the 80s to describe these terms by the free software community, but only as far as free licenses are concerned.
- The GNU Project's General Public License (GPL) and Free Documentation License (GFDL) are similar to share-alike licenses.
- The Creative Commons suite of licenses includes a wider range of share-alike licenses which are denoted usually as "-sa" licenses, e.g.
By contrast, there are also many permissive free software licences which do not require share-alike terms to be applied, thus permitting users to make modifications and improvements and apply a modified and more restrictive license.
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