Philadelphia colloquially referred to as Philly, is the most populous city in the U.S. state of Pennsylvania. It is the sixth-most populous city in the United States with a population of 1.6 million at the 2020 census, while the Philadelphia metropolitan area (sometimes called the Delaware Valley) with 6.33 million residents is the nation's ninth-largest metropolitan area. Philadelphia is known for cuisine, and history, maintaining contemporary influence in business and industry, culture, sports, and music.Philadelphia is a city of many firsts, including the nation's first library (1731),hospital (1751), medical school (1765), national capital (1774),university (by some accounts) (1779),stock exchange (1790), zoo(1874), and business school (1881). Philadelphia contains 67 National Historic Landmarks, including Independence Hall. From the city's 17th century founding through the present, Philadelphia has been the birthplace or home to an extensive number of prominent and influential Americans.