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Within media fandom, pan-fandom refers to media fandom overall, in a meta sense -- not just the sum total of all possible fandoms (and fans), but also things like zine-based fandom, cons, fanfic in general, fanart in general, widespread story tropes, vidding, fannish lingo, etc.
Thus, spaces that encourage interaction among fans without focusing on individual fandom preferences would be pan-fandom spaces (e.g. Fandom Wank, Metafandom, the Fanfic Symposium, FCA-L). Similarly, Robin Hobb's Fan Fiction Rant is a pan-fandom rant, in that it talks about all fanfic writers everywhere.
Contrast this term with multifandom, which refers to multiple individual fandoms, while leaving out the more meta aspects.