Chicago ( /ʃɨˈkɑːɡoʊ/ (help·info) or /ʃɨˈkɔːɡo ʊ/) is the third largest city in the United States, and with more than 2.8 million people, the largest city in the state of Illinois and the Midwestern United States. Located on the southwestern shores of Lake Michigan and next to Indiana, Chicago is the third-most densely populated major city in the U.S., and anchor to the world's 26th largest metropolitan area with over 9.6 million people across three states. The Chicago-Naperville-Joliet MSA also grew more than an estimated 400,000 people between April 1, 2000 and July 1, 2006. Except for the southwest corner of O'Hare International Airport in DuPage County, the city of Chicago is located in Cook County.

Chicago was founded in 1833, near a portage between the Great Lakes and the Mississippi River watershed. The Potawatomi were forcibly removed from their land following the Treaty of Chicago. The city became a major transportation and telecommunications hub in North America. Today, the city retains its status as a major hub, both for industry and infrastructure, with Chicago-O'Hare International Airport as the second busiest airport in the world. In 2007, the city attracted 32.8 million domestic visitors and about 1.15 million foreign visitors.

In modern times, the city has taken on an additional dimension as a center for business and finance and is listed as one of the world's top ten Global Financial Centers. Chicago is a stronghold of the Democratic Party and has been home to influential politicians, including the current President of the United States, Barack Obama. The World Cities Study Group at Loughborough University rated Chicago as an "alpha world city" due to Chicago's important role in the global economic system.

Globally recognized, Chicago has numerous nicknames, which reflect the impressions and opinions about historical and contemporary Chicago. The best known include: "Chi-town" and the "Windy City" with reference to Chicago politicians and residents boasting about their city; "Second City," due to the city's rebirth after the Great Chicago Fire of 1871, although often also attributed to generally being the second largest city in the nation at the time and the second most prestigious in the nation in terms of culture, entertainment, and finance, and because for much of the twentieth century Chicago's population was the second largest of any city in the United States; and the "City of Big Shoulders", referring to its numerous skyscrapers (whose steel frame designs were largely pioneered in Chicago), described as being husky and brawling. Chicago has also been called "the most American of big cities".