A decision rule under which the option preferred by more than half of those voting prevails. With two options it is the only rule that is anonymous, neutral and positively responsive (May's theorem).
In more detail
Majority rule treats voters equally and does not favour either option. With three or more options majorities can cycle, and a plurality is not necessarily a majority. Mill warned in On Liberty of a 'tyranny of the majority', and constitutional systems limit majorities through rights guarantees, courts and supermajority requirements.