The Collaborative International Dictionary of English v.0.48:

minor league \minor league\ n. Sport)
   A league of professional sports teams less proficient than a
   major leagues. Players in the minor leagues generally are
   paid less than those in the major leagues, and their games
   attract less atention.

   Note: Certain minor league sports teams are often owned by
         those owning a major league club, and the minor league
         clubs are used to provide practise and opportunity for
         evaluation for candidates who wish to play in the major
         leagues.

   Syn: minor league.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

minor league
    n 1: a league of teams that do not belong to a major league
         (especially baseball) [syn: minor league, minors, bush
         league]