As of 2005, there is no universally accepted definition or list, but the following North American professional leagues are almost always defined as major leagues:
a) Major League Baseball (formally founded in 1920 though constituent leagues began cooperation in 1903). MLB is divided into the American League (founded in 1901) and the National League (founded in 1876). The two are effectively merged on an organizational level.
b) The National Basketball Association (founded in 1946). The NBA partially absorbed the rival American Basketball Association in 1976.
c) The National Football League (founded in 1920). The NFL partially absorbed the All-America Football Conference in 1949 and merged with the American Football League in 1970.
d) The National Hockey League (founded in 1917). The NHL was founded in 1917 and partially absorbed the rival World Hockey Association in 1979.
Those who define only the first three leagues listed above as major leagues often refer to them as the Big Three.
However, it appears most Americans (and nearly all Canadians) also include the latter league to create a Big Four:
Since the four leagues listed above are those listed as the major leagues, the sports they play (baseball, basketball, American football and ice hockey) are often referred to as the four major professional sports or even just the major sports by North Americans.