The term is derived from the creator of Buffy the Vampire Slayer, Joss Whedon. The term is most prevalent in the science fiction genre.
It is used most commonly when authors are led to believe that a certain event may happen later in a Television series or book and write a story with that event in it, then the official author goes in a different direction, making their story 'AU', Alternate Universe.
Person 2: More than Coulson, less than Connor.
Name comes from Buffy/Angel fanficiton due to the fact that the shows' creator Joss Whedon constantly did this, causing considerable irritation to fans and fanficcers worldwide.
Come season 3 or 4 this is changed to Drusilla being his sire.