The GM is always right. No matter the Dice rolls he wins, The GM always wins
Player: W00t! I rolled a crit again! your Dragon is dead in two hits!
Gm: No it isn't.
Player: What? Why not?
Gm: GM Fiat...
Magical Tea Party - a slightly derogatory term used in roleplaying system design (mostly on The Gaming Den) that refers to a freeform style of RP. Events and outcomes of the players' actions are determined by DM fiat rather than by a character's skill or chance of success.
DM: So as the DM, I decide everything that happens. There will be no dice rolling. What I say goes, and you can react however you like. What happens due to your reaction though is completely at my whim as well.
Player: So... it's basically MTP?
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.
A bad dude/thing in a game that can only be beaten with a specific, usually obscure tactic or item.
Trolls, the classic puzzle monster of D&D, can only be killed by fire and acid. If you don't have those, you're basically screwed.