Chicago, commonly referred to as 'The Windy City', is a major city located in the U.S. state of Illinois. It is the third most populous city in the United States and is the county seat of Cook County. Chicago is known for its iconic skyline, its vibrant culture, and its many attractions, including the Willis Tower, Navy Pier, and Millennium Park. The term 'CHI-Town' is often used to refer to the city of Chicago.