Philadelphia, colloquially referred to as Philly, and known as The City of Brotherly Love  ("brotherly love" from philos "loving" and adelphos "brother") is the fifth most populous city in the United States and the largest in population and area in the Commonwealth of Pennsylvania.

The population of the city (at the 2000 census) was 1,517,550. According to the 2005 U.S. Census estimates, the city had a population of 1,463,281. Philadelphia is the second-largest city on the U.S. East Coast (after New York City), and a major commercial, education, and cultural center for the nation.

The Philadelphia metropolitan area is the fourth largest in the U.S. by the current official definition, with some 5.823 million people.

Philadelphia is one of the oldest and most historically significant cities in the United States. During part of the 18th century, the city was the second capital and most populous city of the United States, and the second largest English speaking city in the world after London. At that time, it eclipsed Boston and New York City in political and social importance, with Benjamin Franklin playing an extraordinary role in Philadelphia's rise.

Before Europeans arrived, the Delaware (Lenape) Indian town of Shackamaxon was located where Philadelphia now stands, specifically the Germantown neighborhood. Although the area lay within the bounds described in the 1632 Charter of Maryland, the Calvert family's influence never reached this far north, and the first European settlers were mostly Swedes (see New Sweden), who called it Wiccacoa. A congregation was formed in 1646 on Tinicum Island by Swedish missionary Johannes Campanius; in 1700, the group built Gloria Dei Church, also known as Old Swedes'.

Philadelphia is a planned city, founded and developed in 1682 by William Penn, a Quaker. The city's name means "brotherly love" in Greek (Φιλαδέλφια). Penn hoped that the city, as the capital of his new colony founded on principles of freedom and religious tolerance, would be a model of this philosophy. During early immigration by Quakers and others, immigrants who purchased land in the city also received farmland outside the city; this was intended to allow the population to leave the city easily. Penn also mandated the construction of alleyways and open spaces, in the hope of controlling fires and disease, which were then common problems in London and other major cities.

Philadelphia was a major center of the independence movement during the American Revolutionary War. The Declaration of Independence and US Constitution were drafted here and signed in the city's Independence Hall. Tun Tavern in the city is traditionally regarded as the location where, in 1775, the United States Marine Corps was founded.

During the American Revolutionary War Philadelphia's population was split between Loyalists and Patriots. When the British Army took the city in 1777 many people lined the streets and sang 'God Save the King'. Upon the retaking of the city for the American cause in 1778 it was the turn of the Patriot population to line the streets in celebration, especially as the population had suffered through a bitter winter with many of the provisions going to the British Army. The subsequent harsh treatment of the Loyalists who had not fled the city was further suffering for Philadelphians.

For a time in the 18th century, Philadelphia was the largest city in the Americas north of Mexico City, and the fourth largest under the rule of the British crown (after London, Bristol, and Dublin).

In 1790, as the result of a compromise between a number of Southern congressmen and Alexander Hamilton, then Secretary of the Treasury, the seat of the United States Government was moved from Federal Hall in New York to Congress Hall in Philadelphia, before assuming its current site in Washington, DC. In exchange for locating a permanent capital on the banks of the Potomac, the congressmen agreed to support Hamilton's financial proposals. Philadelphia served as capital for a decade, until 1800, when the Capitol building in the new federal city of Washington, DC was opened.

The city limits have been coterminous with Philadelphia County since The Act of Consolidation, 1854. Until then, the city consisted only of the area bounded by South and Vine Streets and the Delaware and Schuylkill Rivers. The expansion incorporated present-day West Philadelphia, South Philadelphia, North Philadelphia, and Northeast Philadelphia, as well as Germantown and many smaller communities.

An early railroad center, Philadelphia was the original home of the Baldwin Locomotive Works, the world's largest builder of steam locomotives (which eventually relocated to nearby Eddystone, Pennsylvania). The Pennsylvania Railroad, once America's largest railroad by revenue and traffic volume and at one time the largest public corporation in the world, was headquartered in the city, as was its merger successor, the Penn Central, and in turn its freight railroad successor, Conrail.

In 1876 Philadelphia hosted the World's Fair, known as the Centennial Exposition. Memorial Hall and the expansive mall in front of it are remnants of this fair.

In 1926, the city held the Sesquicentennial Exposition to celebrate the nation's 150th birthday.

In 1976, Philadelphia was one of the participating cities in the United States Bicentennial observances that took place nationwide.

On May 13, 1985 Philadelphia police bombed the Cobb's Creek section of West Philadelphia in order to combat a radical group known as MOVE. City officials allowed the fire to burn until it had consumed 62 area homes and left 11 dead.

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