Or , from Wikipedia:
Since the release of the film, the name Keyser Söze has gained two popular uses in Western culture:
The first is as a description of a legend, usually of underworld crime, which is a result of the character’s presence in The Usual Suspects.
The second is a shorthand reference to being fooled into believing in a person who does not exist. This use of the name is owed to the film’s twist ending.
In his 1999 review of Fight Club, film critic Roger Ebert commented, “A lot of recent films seem unsatisfied unless they can add final scenes that redefine the reality of everything that has gone before; call it the Keyser Söze syndrome.”