= Creative Commons license condition that allows others to distribute derivative works only under a license identical to the license that governs the original work.
"A share-alike copyright license clause requires that any derived version of the work to be shared on like terms with everyone else—that is, "share and share alike".
- The GNU project's General Public License (GPL) and Free Documentation License (GFDL) are share-alike licenses since they require exactly that.
- The Creative Commons suite of licenses includes a wider range of share-alike licenses which are denoted usually as "-sa" licenses, e.g.
- CC-by-sa, which requires Attribution and is generally considered similar to the GFDL
- CC-nc-sa, which requires Noncommercial use only
In the context of Creative Commons only, a share-alike clause states that "If you alter, transform, or build upon this work, you may distribute the resulting work only under a license identical to this one." However, generic variations of share-alike licenses define free software and open content. The term copyleft is also used to describe these terms for free software.
By contrast, many open source and free software licenses (such as the BSD license) do not require share-alike terms to be applied, and permits users to make modifications and improvements and apply a modified and more restrictive license. It is also part of the definition of open source that there can be no restrictions on field of use, which is obviously not the case for all share-alike licenses, e.g. CC-nc-sa."
Critique by David Wiley
"The share-alike requirements of the BY-SA and GFDL licenses are exactly what prohibit material shared under these licenses from interoperating. The interoperability-breaking nature of share-alike requirements is what forced Wikipedia to engage in what might be the largest relicensing of content in the history of humanity – changing its millions of pages of content from the GFDL to a CC license. This had to happen because share-alike requirements destroy interoperability with content licensed under similar (but different) licenses with share-alike requirements. Read that sentence again. I’ll wait for you. If share-alike requirements promote interoperability, I’ll eat hay with a horse. Share-alike requirements only promote interoperability of content using exactly the same license. Is that really anything to be proud of or write home about?
Here I state the SA Fallacy:
1. Share-alike requirements supposedly promote interoperability.
2. Only content licensed under exactly the same share-alike requirement-bearing license is interoperable.
3. Any two pieces of content licensed under exactly the same license would be interoperable even if that license didn’t have a share-alike requirement.
4. THEREFORE, a share-alike requirement in a license does absolutely nothing to promote interoperability.
What is it, then, that share-alike requirements do? Nothing more than prevent a down-stream user from making her own choices about how to license the derivative works she creates. When authors adopt a share-alike license, they are saying: we value the freedom of content over the freedom of people. They are saying: we prefer (1) that all derivatives of our content remain “free” over the option of (2) giving people the “freedom” to choose how to license the derivative works they create. Share-alike requirements give authors a way to privilege bits and bytes above people." (http://opencontent.org/blog/archives/1498)