A term used in Doctor Who fiction to refer to any piece of work which reuses old monsters or characters purely in an attempt to stir feelings in the groins of sad anal fanboys.
Fan1 "In this new book, the Daleks, the Cybermen and Yartek team up to fight a war against the Doctor, UNIT and the Time-Lords. The Doctor finally defeats The Master on the planet Skaro with help from Leela, K9 and the Sonic Screwdriver."
Fan2 "Sounds like a load of fanwank to me!"
To fill in plot holes or explain away lapses in continuity in fictional works by coming up with (often convoluted) explanations of how it could have happened.
"But David used Sarah's real name even though he never knew her before she changed her identity."
"He could have read her file. He had access to it in episode seventeen and there were a few minutes when he could have flicked through it."
When a nerd attempts to explain the inconsistencies in their favourite TV shows/films with supposedly 'common sense' conjecture based on non-canon material. Especially applicable to Science Fiction.
"Did you know that the Nova class science vessel was introduced to replace the Oberth class vessels in the 2360s?"
"Man, is that true?"
"Erm, don't really know. I think I made it up. Makes sense though. Wait, did I just have a fan wank?"