Alternate universe

A universe that is similar to but not the same as any other given universe - typically because of a few small changes. Any Canon universe, and the Real World, can have AUs.

AUs are typically invoked when canon relationships are mucked with (for instance, the common AU where Aragorn never met Arwen and instead spent all his time during LotR shagging Legolas), when significant events don't happen, or don't happen the same (the AU where the Pevensies don't leave Narnia at the end of Prince Caspian), or where something completely inexplicable has happened (the AU of any canon where all the characters are students at a 'typical Japanese/American/other country high school').

When writing a Fic, it is always a good idea to have a good hard think about whether your story is AU.