Philadelphia, commonly referred to as Philly, is the most populous city in Pennsylvania and the second-most populous city in both the Northeast megalopolis and Mid-Atlantic regions after New York City. It is one of the most historically significant cities in the United States and served as the nation's capital until 1800. Philadelphia is the nation's sixth-most populous city with a population of 1,603,797 as of the 2020 census. Since 1854, the city has been coextensive with Philadelphia County, the most populous county in Pennsylvania. Philadelphia serves as the urban core of the larger Delaware Valley (or Philadelphia metropolitan area), the nation's seventh-largest and one of the world's largest metropolitan regions consisting of 6.245 million residents in the metropolitan statistical area and 7.366 million residents in its combined statistical area. Philadelphia is known for its extensive contributions to American history, especially the American Revolution, and for its contemporary influence in business and industry, culture, sports, and music.