I feel like there are two usages that are helpful and reasonably accurate, depending on context. There is the broad “everything is fanfiction” version, which is just “transformative = fanfiction” — and this is useful when contesting the notion that fanfic isn’t valid, creative, &c. because it is rooted in prior canons. There is also the somewhat narrower definition, which I like better (because I am privileged enough not to have to have the above argument very often.) The narrower definition I use is that fanfiction is a modern phenomenon that came into being precisely because of the cultural shifts surrounding the value of originality: once it became important to have new ideas, and possible to claim them as one’s property, then transformative narratives became a marked category in a way they hadn’t been before — so fanfiction calls back deeply to earlier modes of narrative engagement, and hits some deep spot in us that is ignored/denigrated by Enlightenment (and most post-Enlightenment) thought, but is distinct from them because it has had to carve a path for itself through modernity. I am also personally inclined to think that it’s useful to distinguish fanfiction from e.g. pastiche, insofar as its creators are more horizontally related to their readers, with regard to all kinds of capital (economic and establishment-cultural most of all) but I am less wedded to that part.