midwestern states State of Illinois A midwestern state of the United States of America. The Northeastern border of Illinois is Lake Michigan. Its eastern border with Indiana is all of the land west of the Wabash River, and a north-south line above Post Vincennes, or 87deg31min30secW. Its northern border with Wisconsin is fixed at 42deg30minN. Its western border with Missouri and Iowa is the Mississippi River. Its southern border with Kentucky is the Ohio River. Illinois also borders Michigan, but only via a water boundary in Lake Michigan. LanguaL curation note: US FDA 1995 Code: US17 illinois State of Illinois A midwestern state of the United States of America. The Northeastern border of Illinois is Lake Michigan. Its eastern border with Indiana is all of the land west of the Wabash River, and a north-south line above Post Vincennes, or 87deg31min30secW. Its northern border with Wisconsin is fixed at 42deg30minN. Its western border with Missouri and Iowa is the Mississippi River. Its southern border with Kentucky is the Ohio River. Illinois also borders Michigan, but only via a water boundary in Lake Michigan. LanguaL curation note: US FDA 1995 Code: US17 illinois