Chicago is the largest city in the state of Illinois. With over 2.8 million residents, it is the most populous city in the Midwestern United States and the third most populous city in the USA. Its metropolitan area, commonly named "Chicagoland," is the 28th most populous in the world, home to an estimated 9.7 million people spread across the U.S. states of Illinois, Wisconsin, and Indiana. Chicago is the county seat of Cook County.
Chicago was founded in 1833, near a portage between the Great Lakes and the Mississippi River watershed. Today, the city retains its status as a major hub, both for industry and infrastructure, with O'Hare International Airport being the second busiest airport in the world. In 2008, the city hosted 45.6 million domestic and overseas visitors. As of 2010, Chicago's metropolitan area has the 4th largest Gross Domestic Product (GDP) of all metropolitan areas in the world.
The city is a center for business and finance and is listed as one of the world's top ten Global Financial Centers. The World Cities Study Group at Loughborough University rated Chicago as an "alpha world city". In a 2010 survey collaboration between Foreign Policy and A.T Kearney ranking cities, Chicago ranked 6th just after Paris and Hong Kong. The ranking assesses five dimensions: value of capital markets, diversity of human capital, international information resources, international cultural resources, and political influence. Chicago is a stronghold of the Democratic Party and has been home to many influential politicians, including the current President of the United States, Barack Obama.