GPL (GNU General Public License)
acronymprogramminggovernance & risk
The copyleft licence family from the Free Software Foundation: you may use, study, modify and redistribute the software, provided that anything you distribute built from it carries the same freedoms and ships its source.
Version 2 (1991) is what Linux uses. Version 3 (2007) added terms on software patents and on devices built to refuse modified software, and the kernel deliberately stayed on 2 - a split that persists across the ecosystem. The practical difference from permissive licences is who bears the risk of enclosure: permissive terms let anyone take the code private, copyleft terms do not, and which is preferable depends entirely on whether you are trying to maximise adoption or to guarantee that improvements come back.
Also known as: gnu general public license, gplv2, gplv3, copyleft license