The City of Illinois is the most populous city in the state and is located in the northeastern region. It is the county seat of Cook County and is known as the 'Windy City'. The city is home to many iconic landmarks, such as the Willis Tower, Navy Pier, and Millennium Park. The city is also known for its vibrant culture, diverse population, and rich history. The answer to the crossword puzzle is CHICAGO, a seven-letter word that is the name of the City of Illinois.