Viral license

A viral license, in patent, trademark, and copyright practice, is one that applies identically to all derivative works. The term "viral" is an apt analogy for these licenses, as they are designed to spread widely in an uncontrolled manner. Viral licenses tend to maintain a degree of control over intellectual property by restricting the terms of the license strictly and sub-licensing openly, in contrast to copyright licenses where sub-licensing is usually tightly restricted.

In context of legally binding contracts and licenses, "viral" refers to anything, especially anything memetic, that propagates itself by attaching itself to something else. The viral analogy for licensing can be extended to distinguish between concepts such as open source and free source. Many viral licenses are designed with an aim to promote free content.

Wikipedia itself operates off the GNU Free Documentation License, a type of viral license. This allows forking and mirroring of its content and translation of Wikipedia into other languages, all while still remaining under identical license terms to the original content. Similarly, the Share-alike CC-by-sa license is in common usage among other wiki-based projects.

Although many notable viral licenses promote free content, there are also licenses that don't. For instance, the Creative Commons CC-nc-sa license restricts commercial usage of further modifications. Purely commercial licenses can also incorporate viral characteristics, even if they do not lead to strict viral duplication of the original license.

Copyleft licenses overlap with viral licenses because they have many viral characteristics, but are not necessarily strictly viral licenses themselves. For instance, software licensed under the LGPL can interoperate with software that is not under a copyleft license, without automatically "infecting" that software with copyleft.