The Collaborative International Dictionary of English v.0.48:

majority rule \majority rule\ n.
   The rule or doctrine that the numerical majority[2] of an
   organized group can make decisions binding on the whole
   group; as, our club makes decisions by majority rule..
   Contrasted with unanimous consent, or decision by a
   decree of a single person or small committee forming part
   of an organization.
   [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

majority rule
    n 1: the doctrine that the numerical majority of an organized
         group can make decisions binding on the whole group [syn:
         majority rule, democracy]