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 more than 1.54 million, while the Greater Philadelphia metropolitan area's population of 5.8 million made it the country's fifth largest. The city, which lies about 45 miles southwest of New York 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 the literal meaning of the city's name in Greek (Greek: Φιλαδέλφεια (/pʰilaˈdelpʰeːa/, Modern Greek: /filaˈðɛlfia/) "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 (after London), 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. Many of its larger suburbs such as Chester, Pennsylvania; Wilmington, Delaware; Camden, New Jersey; and Trenton, New Jersey (sometimes included in the New York metropolitan area) have African American majorities. This community has been large since before the Great Migration, and despite area civil rights gains, continues to be affected by poverty and high crime. The area, in common with most of Pennsylvania, also has a very large population of Italian Americans.