It's to do with Joss Whedon, and it's a fanfic term. You write a fanfic story that's faithful to canon at the time you write it -- and then Whedon takes the canon storyline in a completely different direction to that which was expected, and suddenly your story has become AU (alternate universe, not canon-faithful). It happened a lot in Buffy, and Whedon was known to occasionally do it *on* *purpose* to mess with fans' heads.
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It's to do with Joss Whedon, and it's a fanfic term. You write a fanfic story that's faithful to canon at the time you write it -- and then Whedon takes the canon storyline in a completely different direction to that which was expected, and suddenly your story has become AU (alternate universe, not canon-faithful). It happened a lot in Buffy, and Whedon was known to occasionally do it *on* *purpose* to mess with fans' heads.