Noun

1. An association of sports teams which plays at the highest skill level of its sport, especially Major League Baseball.

Adjective

1. Of or pertaining to a major sports league.
2. Analogous in importance to a major league sports team.

   That rock band is really major league.

   Calatrava is a major league architect.

3. Serious, intensive.

   The company made a major league effort to get the contract.