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.
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