The GPL is strategically ambiguous as to the meaning of the word "program" and "combine". The licensing terms could be taken to be relative to UNIX concepts lik

The GPL is strategically ambiguous as to the meaning of the word "program" and "combine". The licensing terms could be taken to be relative to UNIX concepts like processes, since Stallman had a very UNIX-centric worldview; but the FSF has been very clear that they consider "program" to be more expansive a definition than that[0]. While there are some cases in which two GPL programs could be said to live in the same address space[1] (or PHP interpreter), WordPress plugins are very much designed to integrate with WordPress and modify its behavior. From a copyright perspective, that smells awfully like a derivative work, and thus all the usual GPL licensing language and copyleft would apply here. Ergo, I'm writing these opinions under the assumption that WordPress plugins have to also be GPL. [0] For example, they consider proxying out to a separate network service to not escape the bounds of a GPL "Program". [1] It's very rare nowadays to see separate programs live in the same address space. Classic Mac OS did this; but I don't think anyone is going to take someone to court over retrocomputing hobbyist software. Linus argues that you can have proprietary kernel modules as long as they only touch user-mode API stuff, but that's because they have a specific licensing exception that makes the UAPI outside the bounds of the GPL copyleft. The web is the only place where you could have GPL Programs coexist within the same isolation boundary.