The free license for software modification on the condition that the same license remains on the product.
A General Public License (GPL) is the license attached to software which allows viewing, use, and even modification of the software provided that the same license remains on the software. Referred to as copyleft, this license category does not forbid copying of the specifications of computer programs. Instead, it encourages widespread use of the software and even possible alterations of their properties. However, when users modify programs with GPL, they have to distribute them under the same license, thereby giving the original developers due credit for their work.
GPL is originally derived from the GNU Project. Thus, it is one of the foundations for the development of open-source software.