GPL
also: GNU General Public License, GPLv2, GPLv3, Copyleft
The GNU General Public License (GPL) is a free software license that guarantees users the freedom to use, modify, and distribute software, with the requirement that derivative works remain under the same license.
The GPL is one of the most widely used open-source software licenses. It grants users four essential freedoms: to run the software for any purpose, to study how it works, to modify it, and to redistribute copies—both modified and unmodified versions.
The GPL's defining characteristic is copyleft: if you distribute a modified version of GPL software, you must release your changes under the same GPL license. This ensures the software remains free and open for all downstream users. For example, if you modify the Linux kernel and distribute it, your modifications must also be GPL-licensed.
The GPL comes in versions (GPLv2, GPLv3, etc.), with GPLv3 providing additional protections against software patents and restrictions. Most core Linux components, including the kernel itself, use GPLv2.