GNU General Public License

This license, commonly known as the GPL, has two versions that are actively and widely used in many open source communities:

If you have licensed software you've written under GPL version 2, and you are the original licensor of that software, you may wish to relicense your software under GPL version 3.

Most GPL version 2 software is actually licensed as "GPL version 2 or any later version", or words to similar effect, which is commonly understood as a grant under GPL version 2, section 9 of an "option to follow the terms and conditions . . . of any later version" of the GPL. In this case, even if you are not the original licensor of the GPL version 2 software, you can "follow the terms and conditions" of GPL version 3 instead. This is not commonly done (at least in any explicit way), however.

To promote and protect open source software and communities...

For over 20 years the Open Source Initiative (OSI) has worked to raise awareness and adoption of open source software, and build bridges between open source communities of practice. As a global non-profit, the OSI champions software freedom in society through education, collaboration, and infrastructure, stewarding the Open Source Definition (OSD), and preventing abuse of the ideals and ethos inherent to the open source movement.

Open source software is made by many people and distributed under an OSD-compliant license which grants all the rights to use, study, change, and share the software in modified and unmodified form. Software freedom is essential to enabling community development of open source software.