Rule zero refers to the ground foundation of every roleplaying game; the base principle, that highlights that the Dungeon Master (DM), also known as Game Master (GM) is always right, and always has a right to veto anything in the game.
“Rule zero” is also referred to among the players as the zero-th rule.
Every other rule comes after this, if the GM approves of it.
Origin of the term
The history of tabletop roleplaying games goes back in time to chess, which then evolved into various war games, and finally in 1971 the first miniature wargame, titled Chainmail.
Chainmail had served as a foundation for Dungeons and Dragons, that was first published in 1974.
Role playing games became more and more of a phenomenon, thus people who didn’t know anything about the method of playing, needed a ground rule to be established; the Game Master is always right.
Spread of the term
This traditional principle had stuck among roleplayers, who are still using the expression “Rule Zero” before rushing into the quests, they gathered to do.
While tabletop games are massively being replaced by their online peers, there are still many people, who are delving deep into dungeons for long hours with their friends.
The phrase was first defined on Urban Dictionary in 2007.