Chicago, a city in the U.S. state of Illinois, is the third most populous city in the United States and the most populous city in the American Midwest, with approximately 2.7 million residents. Its metropolitan area (also called "Chicagoland"), which extends into Indiana and Wisconsin, is the third-largest in the United States, after those of New York City and Los Angeles, with an estimated 9.8 million people. Chicago is the county seat of Cook County, though a small portion of the city limits also extends into DuPage County.

Chicago was incorporated as a city in 1837, near a portage between the Great Lakes and the Mississippi River watershed. Today, Chicago is listed as an alpha+ global city by the Globalization and World Cities Research Network, and ranks seventh in the world in the 2012 Global Cities Index. The city is an international hub for finance, commerce, industry, telecommunications, and transportation, with O'Hare International Airport being the second-busiest airport in the world in terms of traffic movements.

In 2012, Chicago hosted 46.2 million international and domestic visitors. Among metropolitan areas, Chicago has the fourth-largest gross domestic product (GDP) in the world, just behind Tokyo, New York City, and Los Angeles, and ranking ahead of London and Paris. Chicago is one of the most important Worldwide Centers of Commerce and trade.

Chicago's notability has found expression in numerous forms of popular culture, including novels, plays, films, and songs. The city has many nicknames, which reflect the impressions and opinions about historical and contemporary Chicago. The best-known include "Windy City" and "Second City.