The Collaborative International Dictionary of English v.0.48:

major power \major power\ n.
   A nation powerful enough to influence events throughout the
   world.

   Syn: world power, great power, power.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

major power
    n 1: a state powerful enough to influence events throughout the
         world [syn: world power, major power, great power,
         power, superpower]