Fictional Fans

For me it's a "I know it when I see it" kind of thing, like most genres.

I think recognition works on a kind of fuzzy setting, where there are a bunch of criteria (legal status, typical tropes, (non)profit motive of the writer, community with audience, derivative from other source...) and if it fits enough of those it's fanfic, but not every fanfic has to fit the platonic ideal of fanfic that's prototypical and really fits every marker you can think of.