1a major river in North America, one of the main tributaries of the Mississippi River. It rises in the Rocky Mountains in Montana and flows 2,315 miles (3,736 km) to meet the Mississippi River just north of St. Louis. 2a state in the central part of the US, bounded on the east by the Mississippi River; population 5,911,605 (est. 2008); capital, Jefferson City; statehood, Aug. 10, 1821 (24). It was acquired as part of the Louisiana Purchase in 1803 and admitted as a state as part of the Missouri Compromise.