A major professional league, usually simply reffered to as a major league, is a league that is considered the best in a given country. Major leagues usually have access to considerable amounts of cash and therefore can establish highly competitive teams filled with high quality personnel in large cities, unlike minor leagues, who usually establish themselves in smaller markets.
Major leagues are less common than minor leagues, and there is usually only one per country. An exception however dates back to the 1970's, when the National Hockey League and the World Hockey Association both declared themselves major leagues and competed for the best players and bigger markets in North America.