What is a viral license?

A viral license is one that requires any modifications or additions to the licensed work to be released under the same license. This can have the effect of “infecting” derivative works with the same licensing requirements.

For example, the popular open source license GPL (GNU Public License) is a viral license, because any derivative works must also be released under the GPL.

This type of license is also known as a copyleft license, since it requires users to keep the software free and open source.