There are many open source licenses, the GPL being one of the most popular and the OSI (The Open Source Initiative) has an approval process for open source licenses with detailed information on their website but essentially open source licenses must ensure that the code is available for viewing and changing and that the software can be redistributed in it original or modified form. The GPL (GNU General Public License) now in version 3 is one of the most well known and used open source licenses.

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