A film or book that has a strong but typically confusing plot, up to the end where it's revealed that much of what the main character(s) and/or audience believed to be true is an illusion of some sort. It was good with Fight Club, but after a while it tends to be disappointing once you realise that it's an age old plot. Examples include Fight Club, The Machinist, Secret Window, Matchstick Men, and probably some other stuff I'm forgetting.
Named after a Dissociative Fugue State of amnesia, in which the Merck Manual of Diagnosis and Therapy defines as "One or more episodes of amnesia in which the inability to recall some or all of one's past and either the loss of one's identity or the formation of a new identity occur with sudden, unexpected, purposeful travel away from home.", and further "Dissociative Amnesia" as "An inability to recall important personal information, usually of a traumatic or stressful nature, that is too extensive to be explained by normal forgetfulness."
That movie was alright up until it turned out that the creepy fat guy turned out to be made up in the main characters head. Then I realised it was just another fugue film.