Philadelphia is the largest city in Pennsylvania, sixth-most-populous city in the United Statesand the fifty-first most populous city in the world.

It is the county seat of Philadelphia County, with which it is coterminous. Popular nicknames for Philadelphia include Phillyand The City of Brotherly Love, from the literal meaning of the city's name in Greek (Greek: Φιλαδέλφεια ([pʰilaˈdelpʰeːa],Modern Greek: [filaˈðelfia]) "brotherly love", compounded from philos (φίλος) "love", and adelphos (ἀδελφός) "brother").

A commercial, educational, and cultural center, Philadelphia was once the second-largest city in the British Empire (afterLondon), and the social and geographical center of the original 13 American colonies. It was a centerpiece of early American history, host to many of the ideas and actions that gave birth to the American Revolution and independence. It was the most populous city of the young United States, although by the first census in 1790, New York City had overtaken it. Philadelphia served as one of the nation's many capitals during the Revolutionary War and after. After the ratification of the U.S. Constitution, the city served as the temporary national capital from 1790 to 1800 while Washington, D.C., was under construction.

Philadelphia is central to African American history; its large black population predates the Great Migration.