Colorado is a state in the western United States. Colorado is best known as the home of the highest peaks of the Rocky Mountains, which dominate the western half of the state; eastern Colorado is largely plains. The state capital and largest city is Denver, Colorado; the Denver metropolitan area contains over half of the state's population (2.5 million out of 4.3 million). The state is named for the Colorado River, which takes its name from the Spanish word "Colorado" which means "reddish color". The U.S. Post Office abbreviation for the state is CO. USS Colorado was named in honor of this state.