The term "derivative work" (or similar) is sometimes defined in law, but of course very abstractly (i.e. not specific to software) and differently depending on jurisdiction. (See https://en.wikipedia.org/wiki/Derivative_work) IMO it's exactly because of this ambiguity that many developers' understanding of the term is likely to be incorrect, and that most of us don't really understand what our license choice entails.

I think a better way to say this is that every developer's understanding is absolutely incorrect in some jurisdictions. Maybe they're correct in their jurisdiction, but once you start talking about this online, as with every discussion of legalities on HN, people start talking past each other forgetting that many European countries will disagree with each other, and especially forgetting that the US may have several dozen wildly different interpretations.

Yes , exactly my point. The vagueness is intentional to cause this level of confusion.