A rule of table top gaming. If the DM/GM is being an Ass/dumbass/idiot and/or abusing rule 0, the players can choose to override him by leaving his game and starting their own.
An alternate version involves Smacking the DM over the head with the Dungeon Master's Guide.
DM: The ghoul rolled a 28 On his check to resist your Trip attempt.
Player: How the Hell does a ghoul get a 28?
Dm: With His +8 grapple modifier.
Player: But it is a Strength check, not a grapple!!
DM: I am the DM, Under Rule 0 I say He uses his Grapple Modifier.
This is where Rule 0.5 Is invoked
Every player at table: Smacks DM upside head with books and Leaves game.
A Video game Developer/Publisher. Responsible for Many truly epic games, which also have truly epic bugs. Lists of bugs for their games are dozens of pages long, and their patches often leave the games worse off than before. Currently the Reigning king of the unpolished game.
DUDE 1: I can't wait For the new game by Bethesda Software!!!
DUDE 2: I can wait. They never finish their games until a few months after release. Why should I pay them to be a Beta tester.
1: A GM/DM who Lives and Dies By rule 0. Often cannot do anything without invoking it.
2: A Member of RPG forums who uses rule 0 all the time In rules discussion, contributing absolutely nothing to the Thread. Often uses it as an excuse to justify pushing his personal feelings as the right and proper thing to do.
Normal person: According to The designers of the book, It is acceptable to do this.
0tard's response: Well I do it this way, And I can because of rule 0, and I think everyone should too.
A performer who's sexuality is determined by her fanbase.
Person A: What is Lady GaGA's sexual orientation again?
Person B: Let me check what her fan base wants right now.
1: any white man who does not agree with absolutely everything Obama says or does.
2: anyone who does not constantly kiss Jessie Jackson's backside.
Obama's tie does not match his tie very well. Racist: You racist cracker Asshole!!! how dare you criticize the first black president!