Elusive 'rule' or 'law' that can be invoked to carry through the will of the majority. The last word in all decision making processes, whether this be in persuading others to go on a lovely trip to the park, or something altogether more serious. If the majority of people agree about something, e.g. going to the park, and someone says 'Majority Rules' then everyone must do what the majority want. In the same way, can also be used to prove or disprove facts (however ridiculous), in that if the majority agree that something is true/untrue then it is true/untrue - the 'rule' is thus useful for winning arguments where you know others will agree with you.
Arguably pretty much just democracy distilled to the level of petty playground rivalry.