Aurora, Colorado. Aurora (/əˈrɔərə/, /əˈrɔːrə/) is a Home Rule Municipality in the U.S. state of Colorado, spanning Arapahoe and Adams counties, with the extreme southeastern portion of the city extending into Douglas County. Aurora is one of the principal cities of the Denver-Aurora-Lakewood, CO Metropolitan Statistical Area (Metro Denver). The city's population was 325,078 in the 2010 census, which made it the third most populous city in the state of Colorado and the 54th most populous city in the United States.