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In the context of television fandom, a casual fan is usually someone who enjoys watching the show, but doesn't engage much, if at all, with the fandom. Casual fans have the advantage of not being emotionally devastated if the show jumps the shark or if a favorite ship does not become canon. "Casual" is relative and is sometimes used to describe a fan's attitude toward one particular canon rather than their relationship to fandom overall, e.g. they might be a casual fan of Battlestar Galactica, but heavily involved in Stargate SG-1 fandom.
Google search results show that the term is also in use among sports fans.