Chicago is the most populous city in the U.S. state of Illinois and in the Midwestern United States. Located on the western shore of Lake Michigan, it is the third-most populous city in the United States, with a population of 2.74 million at the 2020 census. The Chicago metropolitan area has 9.41 million residents and is the third-largest metropolitan area in the country. Chicago is the seat of Cook County, the second-most populous county in the U.S.

Chicago was incorporated as a city in 1837 near a portage between the Great Lakes and the Mississippi River watershed. It grew rapidly in the mid-19th century. In 1871, the Great Chicago Fire destroyed several square miles and left more than 100,000 homeless, but Chicago's population continued to grow. Chicago made noted contributions to urban planning and architecture, such as the Chicago School, the development of the City Beautiful movement, and the steel-framed skyscraper.

Chicago is an international hub for finance, culture, commerce, industry, education, technology, telecommunications, and transportation. It has the largest and most diverse finance derivatives market in the world, generating 20% of all volume in commodities and financial futures alone. O'Hare International Airport is routinely ranked among the world's top ten busiest airports by passenger traffic, and the region is also the nation's railroad hub. The Chicago area has one of the highest gross domestic products (GDP) of any urban region in the world, generating $689 billion in 2018. Chicago's economy is diverse, with no single industry employing more than 14% of the workforce.

Chicago is a major destination for tourism, with 55 million visitors in 2024 to its cultural institutions, Lake Michigan beaches, restaurants, and more. Chicago's culture has contributed much to the visual arts, literature, film, theater, comedy (especially improvisational comedy), food, dance, and music (particularly jazz, blues, soul, hip-hop, gospel, and electronic dance music, including house music). Chicago is home to the Chicago Symphony Orchestra and the Lyric Opera of Chicago, while the Art Institute of Chicago provides an influential visual arts museum and art school. The Chicago area also hosts the University of Chicago, Northwestern University, and the University of Illinois Chicago, among other institutions of learning. Professional sports in Chicago include all major professional leagues, including two Major League Baseball teams. The city also hosts the Chicago Marathon, one of the World Marathon Majors.

The name Chicago is derived from a French rendering of the indigenous Miami–Illinois name Šikaakonki, the locative form of the word šikaakwa which can mean both "skunk" and "ramps," a wild relative of onion and garlic known to botanists as Allium tricoccum. The first known reference to the site of the city of Chicago as "Checagou" was by Robert de LaSalle around 1679 in a memoir. Henri Joutel, in his journal of 1688, noted that the eponymous wild "garlic" grew profusely in the area. According to his diary of late September 1687:

... when we arrived at the said place called "Chicagou" which, according to what we were able to learn of it, has taken this name because of the quantity of garlic which grows in the forests in this region.

The city has had several nicknames throughout its history, such as the Windy City, Chi-Town, Second City, and City of the Big Shoulders.

In the mid-18th century, the area was inhabited by the Potawatomi, an indigenous tribe who had succeeded the Miami, Sauk and Meskwaki peoples in this region.