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. With an estimated population of 2,693,976 in 2019, it is also the most populous city in the Midwestern United States. 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 or the Combined Statistical Area, often called Chicagoland. It constitutes the third most populous urban area in the United States after New York City and Los Angeles.