The Collaborative International Dictionary of English v.0.48:

major league \major league\ n. (Sports)
   The most important league[2] in any sport (especially
   baseball); contrasted with minor league. Its teams are more
   competent, its members are paid more, and its games attract
   more interest and publicity, than those of a minor league.
   [WordNet 1.5 +PJC] major-league

The Collaborative International Dictionary of English v.0.48:

major-league \major-league\, major league \major league\a.
   1. Of or pertaining to a major league; as, major-league
      football rules.
      [PJC]

   2. Among the best or most important of its class; as, a
      major-league computer manufacturer.
      [PJC] major-league club

WordNet (r) 3.0 (2006):

major league
    n 1: the most important league in any sport (especially
         baseball) [syn: major league, big league, majors]

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "major league":
   big, big-league, big-name, big-time, bigwig, bigwigged,
   consequential, considerable, double-barreled, earthshaking, grand,
   great, heavyweight, high-powered, important, major, material,
   momentous, name, self-important, significant, substantial,
   superior, world-shaking