Philadelphia (pronounced /ˌfɪləˈdɛlfiə/) is the largest city in Pennsylvania and the sixth-most-populous city in the United States.

In 2008, the population of the city proper was estimated to be over 1.54 million, while the Greater Philadelphia metropolitan area's population of 5.8 million made it the country's fifth-largest. The city is the nation's fourth-largest urban area by population and its fourth-largest consumer media market as ranked by the Nielsen Media Research. It is the county seat of Philadelphia County (with which it is coterminous). Popular nicknames for Philadelphia include Philly and The City of Brotherly Love, from Greek: Φιλαδέλφεια, pronounced [pʰilaˈdelpʰeːa], Modern Greek: [filaˈðɛlfia], "brotherly love" from φίλος, (philos, love), and ἀδελφός (adelphos, brother).

A commercial, educational, and cultural center, the city was once the second-largest in the British Empire (after London), and the social and geographical center of the original 13 American colonies. Ben Franklin took a large role in Philadelphia's early rise to prominence. It was in this city that many of the ideas, and subsequent actions, gave birth to the American Revolution and American Independence, making Philadelphia a centerpiece of early American history. 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. Following the ratification of the U.S. Constitution, it was the temporary national capital from 1790 to 1800 while Washington, DC was under construction.