Illinois is a state in the Midwestern United States. Its largest metropolitan areas include the Chicago metropolitan area, and the Metro East section, of Greater St. Louis. Other metropolitan areas include Peoria and Rockford, as well as Springfield, its capital. Of the fifty U.S. states, Illinois has the fifth-largest gross domestic product, the sixth-largest populati…Illinois is a state in the Midwestern United States. Its largest metropolitan areas include the Chicago metropolitan area, and the Metro East section, of Greater St. Louis. Other metropolitan areas include Peoria and Rockford, as well as Springfield, its capital. Of the fifty U.S. states, Illinois has the fifth-largest gross domestic product, the sixth-largest population, and the 25th-largest land area.