Share-alike is a descriptive term used in the Creative Commons project for copyright licenses which include certain copyleft provisions.

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 a license identical to this one." 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.
• CC-by-sa, which requires attribution and is similar but incompatible to the GFDL
• CC-by-nc-sa, which requires non-commercial use only is share-alike but is not free content, and thus cannot be considered a Copyleft license.

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.