I'd start from the defintion of Free Software per GNU: https://www.gnu.org/philosophy/free-sw.html#four-freedoms https://www.gnu.org/philosophy/free-sw.html#fou

I'd start from the defintion of Free Software per GNU: https://www.gnu.org/philosophy/free-sw.html#four-freedoms https://www.gnu.org/philosophy/free-sw.html#four-freedoms Personally I consider GPL an _implementation_ of copyleft rather than copyleft itself.