Mmm, I get what you're saying, but I don't agree.

1. Fusion is a word to describes a process, in both cases. It's not a noun made up of multiple words like hot dog. And 2) Science terms in the 21st century are coined to be unambiguous. That's the point of defining something in science.

If we want to get technical, it's nuclear fusion. But that still leaves the idea you could be discussing 'nuclear' melting, which is again, ambiguous. Science doesn't like ambiguity.