Chicago, officially the City of Chicago, is the third-most populous city in the United States. With over 2.7 million residents, it is the most populous city in the state of Illinois and the Midwestern United States, and the county seat of Cook County. The Chicago metropolitan area, often referred to as Chicagoland, has nearly 10 million people and is the third-largest in the U.S.

Chicago was incorporated as a city in 1837, near a portage between the Great Lakes and the Mississippi River watershed, and grew rapidly in the mid-nineteenth century.

The city is an international hub for finance, commerce, industry, technology, telecommunications, and transportation: 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 2014 Global Cities Index.

Chicago has the third-largest gross metropolitan product in the United States—about $630.3 billion according to 2014–2016 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

sourse: wikipedia