Chicago officially the City of Chicago, is the most populous city in the U.S. state of Illinois, and the third-most populous city in the United States, following New York City and Los Angeles. With a population of 2,746,388 in the 2020 census, it is also the most populous city in the Midwestern United States and the fifth-most populous city in North America. Chicago is the county seat of Cook County, the second-most populous county in the U.S., while a small portion of the city's O'Hare Airport also extends into DuPage County. Chicago is the principal city of the Chicago metropolitan area, defined as either the U.S. Census Bureau's metropolitan statistical area (9.6 million people) or the combined statistical area (almost 10 million residents), and often called Chicagoland.