An old-fashioned term, fringe fandom refers to any specialized subfandom, such as those devoted to television shows, gaming, anime or comics, originally subfamdoms which branched from the original sf fandom that started in the 1930s in the lettercol of Hugo Gernsback's Amazing Stories and the Science Fiction League. It is also sometimes used for less related but overlapping groups such as the Society for Creative Anachronism and mystery fandom, as well as specialized literary groups like the Mythopoeic Society.
The term dates to the 1960s or '70s, when those groups first branched off to hold specialized conventions, and has fallen into disuse, in part because those fandoms now far outnumber the group that concentrates on written sf, sf fanzines, Worldcons and old-time clubs, and tend to resent the implication that there is Trufandom that excludes them. We are all supposed to be together under one big tent, which, however, squeezes out fanhistory and other interests of old-style fen.
Controversy erupted after the rise of Star Trek fandom and comics fandom. Although they had established their own conventions and institutions, when their increasing numbers threatened to overwhelm Worldcon, and the concom of MidAmeriCon in 1976 announced that there would be no programming related to those topics at the con, there was a huge uproar.