Page "Libre" Paragraph 31

Copyleft describes a requirement on some libre licences that copies and modifications of the original work must be available under the same or similar licence.

In this way, copyleft licences guarantee that all modifications and extensions of a libre work will be free as well.

The GNU General Public License was the first copyleft licence and remains the most commonly used.

When a libre licence has a share-alike term, it is a copyleft licence.

For example, the Creative Commons Attribution-ShareAlike licence is a copyleft licence.