In Dungeons and Dragons Third Edition Rule 0 states, "The Dungeon Master can override any other rule." It's a fancy way of saying, "The DM is always right." You'll often see this term used as a verb, "Well, if you don't like how the magic item creation rules work, you can always rule 0 it." (Verbing is common in RPGs.)

This basic idea exists in almost every roleplaying game on the market. The only time where you cannot rule 0 an unliked piece of rules out of a game you are running is when it's not your game, such as the Living campaigns of the RPGA or when you're guest-GMing for a friend.