Chicago is the third-most populous city in the United States. With over 2.7 million residents, it is also the most populous city in both the state of Illinois and the Midwestern United States. It is the county seat of Cook County. The Chicago metropolitan area, often referred to as Chicago land, has nearly 10 million people and is the third-largest in the U.S. Chicago has been called a global architecture capital. In terms of wealth and economy, Chicago is considered one of the most important business centres in the world. The city has many nicknames, the best-known being the Windy City

Chicago was incorporated as a city in 1837, near a portage between the Great Lakes and the Mississippi River watershed. O'Hare International Airport is the second-busiest airport in the world when measured by aircraft traffic; the region also has the largest number of U.S. highways and rail road freight. In 2012, Chicago was listed as an alpha global city by the Globalization and World Cities Research Network, and ranked seventh in the world in the 2016 Global Cities Index. Chicago has the third-largest gross metropolitan product in the United States about $640 billion according to 2015 estimates. The city has one of the world's largest and most diversified economies with no single industry employing more than 14% of the workforce.