Chicago is the largest city in the US state of Illinois. With nearly 2.7 million residents, it is the most populous city in the Midwestern United States and the third most populous in the USA, after New York City and Los Angeles. Its metropolitan area, sometimes called "Chicago land," is the 27th most populous urban agglomeration in the world, the largest in the Great Lakes Megalopolis, and the third largest in the United States, home to an estimated 9.8 million people spread across the US states of Illinois, Wisconsin, and Indiana. Chicago is the county seat of Cook County, the second most populous county in the United States, after Los Angeles County, California.

Chicago was incorporated as a town in 1833, near a portage between the Great Lakes and the Mississippi River watershed. Today, the city retains its status as a major hub for industry, telecommunications and infrastructure, with O'Hare International Airport being the second busiest airport in the world in terms of traffic movements. In 2008, the city hosted 45.6 million domestic and overseas visitors. As of 2010, Chicago's metropolitan area has the 4th largest Gross Domestic Product (GDP) among world metropolitan areas.

The city is a center for services, business and finance and is listed as one of the world's top ten Global Financial Centers. The World Cities Study Group at Southborough University rated Chicago as an "alpha+ world city." In a 2010 survey collaboration between Foreign Policy and A.T Kearney ranking cities, Chicago ranked 6th, just after Paris and Hong Kong. The ranking assesses five dimensions: value of capital markets, diversity of human capital, international information resources, international cultural resources, and political influence. Chicago has been ranked by The Atlantic as the world's 4th most economically powerful city, and by Forbes as 5th most powerful. Chicago is a stronghold of the Democratic Party and has been home to many influential politicians, including the current President of the United States, Barrack Obama.

The city's notoriety expressed in popular culture is found in novels, plays, movies, songs, various types of journals (for example, sports, entertainment, business, trade, and academic), and the news media. Chicago has numerous nicknames, which reflect the impressions and opinions about historical and contemporary Chicago. The best known include: "Chi-town," "Windy City," "Second City" and the "City of Big Shoulders." Chicago has also been called "the most American of big cities.