Philadelphia, colloquially Philly, is a city in the state of Pennsylvania in the United States. It is the sixth-most-populous city in the United States and the most populous city in the state of Pennsylvania, with a 2019 estimated population of 1,584,064. It is also the second-most populous city in the Northeastern United States, behind New York City. Since 1854, the city has had the same geographic boundaries as Philadelphia County, the most-populous county in Pennsylvania and the urban core of the eighth-largest U.S. metropolitan statistical area, with over 6 million residents as of 2017. Philadelphia is also the economic and cultural center of the greater Delaware Valley along the lower Delaware and Schuylkill rivers within the Northeast megalopolis. The Delaware Valley's 2019 estimated population of 7.21 million makes it the ninth-largest combined statistical area in the United States.
Philadelphia is one of the oldest municipalities in the United States. William Penn, an English Quaker, founded the city in 1682 to serve as capital of the Pennsylvania Colony. Philadelphia played an instrumental role in the American Revolution as a meeting place for the Founding Fathers of the United States, who signed the Declaration of Independence in 1776 at the Second Continental Congress, and the Constitution at the Philadelphia Convention of 1787. Several other key events occurred in Philadelphia during the Revolutionary War including the First Continental Congress, the preservation of the Liberty Bell, the Battle of Germantown, and the Siege of Fort Mifflin. Philadelphia remained the nation's largest city until being overtaken by New York City in 1790; the city was also one of the nation's capitals during the revolution, serving as temporary U.S. capital while Washington, D.C. was under construction.