Creative Commons Attribution-ShareAlike 4.0 International

Copyleft Creative Commons licence. Free reuse with attribution; derivatives must be released under the same licence.

CC BY-SA 4.0 adds a share-alike (copyleft) requirement to the basic CC BY 4.0 terms. Reusers may redistribute and modify the work, including for commercial purposes, but any derivative work must be released under CC BY-SA 4.0 (or a compatible licence).

The share-alike requirement is what makes this licence the choice for projects that want their work to remain in the open-licence ecosystem — Wikipedia is the canonical large-scale example. CC BY-SA 4.0 declares one-way compatibility with the Free Art Licence 1.3 and the GNU FDL, allowing migration in those directions.

For datasets, CC BY-SA can produce thorny questions about what counts as a derivative — particularly in geospatial workflows where a single dataset might be combined with many others. The Open Data Commons ODbL handles the same conceptual problem with explicit data-specific drafting.