What Does AH Mean In Fanfiction?

AH means alternate history. It means that something else happened, for instance, an author writing a fanfiction story in which Harry Potter had been killed when he was a baby. It is very similar to AU, which is alternate universe, on the basis of "what if this had happened".

AH stands for "alternate history". Same characters but different sequence of events that brought them to the time period covered in the work of fiction.