Meaning

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

• (usually in plural or attributive) The highest level of competition in a field of endeavor.

  "You can't be so casual. You're in the major leagues now."

Meaning

• Of or pertaining to a major sports league; belonging to the professional level of a sport, such as US baseball.

• Analogous in importance to a major league sports team.

  "Calatrava is a major league architect."

• Serious, high-level; significant

  "Bill accidentally faxed sensitive documents to our competitors, and such a major league mess-up could only be dealt with by firing Bill."