J.k. rowling's thoughts ghost plots
This is a personal expression, which has nothing to do with tales of the dead.
Over the seventeen years that I planned and wrote the seven Harry Potter books (not to mention Quidditch through the Ages, Fantastic Beasts and Where to Find Them and The Tales of Beedle the Bard), I generated a mass of information about the magical world that never appeared in the books. I liked knowing these things (which was fortunate, given that I couldn't stop my imagination spewing it all out) and often, when I needed a throwaway detail, I had it ready because of the background I had developed.
I also found myself developing storylines for secondary (or even tertiary) characters that were superfluous to requirements. More of a wrench were the plots I worked out for some much more important characters that had to be sacrificed for the bigger story. All of these I inwardly termed 'ghost plots', my private expression for all the untold stories that sometimes seemed quite as real to me as the 'final cut'. I have occasionally been in conversation with a reader and made mention of part of a ghost plot; looks of consternation cross their faces as, for a split second, they ask themselves whether they have accidentally skipped twenty pages somewhere. I apologise to anyone I might have accidentally wrong-footed in this way; the problem is, literally, all in my head.