Claim: In the mid-nineteenth century, Chicago grew quickly.

Chicago -LRB- -LSB- ʃᵻˈkɑːɡoʊ -RSB- or -LSB- ʃᵻˈkɔːɡoʊ -RSB- -RRB- , officially the City of Chicago , is the third-most populous city in the United States . With over 2.7 million residents , it is also the most populous city in both the state of Illinois and the Midwestern United States . It is 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 has been called a global architecture capital . In terms of wealth and economy , Chicago is considered one of the most important business centers in the world .   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 . Positioned along Lake Michigan , 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 2016 Global Cities Index . Chicago has the third-largest gross metropolitan product in the United States -- about $ 640 billion according to 2015 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 .   In 2016 , Chicago hosted over 54 million domestic and international visitors , a new record making it one of the top visited cities in the nation . Landmarks in the city include Millennium Park , Navy Pier , the Magnificent Mile , Art Institute of Chicago , Museum Campus , the Willis -LRB- Sears -RRB- Tower , Museum of Science and Industry , and Lincoln Park Zoo . Chicago 's culture includes the visual arts , novels , film , theater , especially improvisational comedy , and music , particularly jazz , blues , soul , hip hop , gospel and house music . There are many colleges and universities in the Chicago area ; among these , Northwestern University , University of Chicago , and the University of Illinois at Chicago are classified as `` highest research '' doctoral universities . Additionally , Chicago has professional sports teams in each of the major professional leagues . The city has many nicknames , the best-known being the Windy City .