General Public License
amodel agreement, developed by the Free Software Foundation, that establishes terms under which individuals may sell, copy, or modify a product, while protecting the creator's interests
Sometimes referred to as Copyleft. The GPL is typically issued with software, allowing programmers to use the software for free. The standard GPL includes a requirement that any modifications to the code must also be made freely available.
Moody 1997, 4Linus [Torvalds] also adopted the standard GNU licensing scheme called Copyleft. The general public license, or GPL, allows users to sell, copy, and change Copylefted programs—which can also be copyrighted—but you must pass along the same freedom to sell or copy your modifications and change them further. You must also make the source code of your modifications freely available.
Söderberg 2002“Copyleft uses copyright law, but flips it over to serve the opposite purpose instead of a means of privatizing software, it becomes a means of keeping software free” [citing Stallman].