Indianapolis, colloquially known as Indy, is the capital and most populous city of the U.S. state of Indiana and the seat of Marion County. Locat…Indianapolis, colloquially known as Indy, is the capital and most populous city of the U.S. state of Indiana and the seat of Marion County. Located in Central Indiana, the city lies along the White River's West Fork near its confluence with Fall Creek. Indianapolis's official slogan, "Crossroads of America", reflects the city's central location relative to other major North American markets and its historic importance as a transportation hub.