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 of copyright law to refer to a similar plan for patent licensing.
Read more about Share-alike: Copyleft, Creative Commons, History of Creative Commons ShareAlike Versions, Adoption