Chicago is a city.
Chicago ( [ ʃᵻˈkɑːɡoʊ ] or [ ʃᵻˈkɔːɡoʊ ] ) , 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 . 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 . A county seat is an administrative center , seat of government , or capital city of a county or civil parish .