a.k.a. "The GM is always right."
The unwritten rule in tabletop role-playing games (such as Dungeons & Dragons) which grants the game master the right to suspend or override the published game rules whenever s/he deems necessary.
Similar to a house rule
; however, rule zero may be invoked unilaterally, at any time and does not have to be agreed upon in advance by the players.
When abused, can lead to an inconsistent, confusing, or frustrating experience for the players.
Bob: I climb up the wall and into the wizard's castle, bypassing the defenses. I rolled a 20, so I succeed.
GM: The wall is magically warded against climbing. You fail.
Bob: But a 20 is an automatic success!
GM: Rule 0. You fail.