London i/ˈlʌndən/ is the capital city of England and the United Kingdom, and the largest city, urban zone and metropolitan area in the United Kingdom, and the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its founding by the Romans, who named it Londinium. London's ancient core, the City of London, largely retains its square-mile mediaeval boundaries. Since at least the 19th century, the name London has also referred to the metropolis developed around this core. The bulk of this conurbation forms the London region and the Greater London administrative area, governed by the elected Mayor of London and the London Assembly.
The City of London is a district of London. The City constituted most of London from its settlement by the Romans in the 1st century AD to the Middle Ages, but the conurbation has since grown far beyond its borders. As the City's boundaries have remained almost unchanged since the Middle Ages, it is now only a tiny part of the metropolis, though it remains a notable part of central London. The City holds city status in its own right, and is also a separate ceremonial county.
It is often referred to as the City (often written on maps as "City") or the Square Mile, as it is just over one square mile (1.12 sq mi (2.90 km2)) in area. These terms are also often used as metonyms for the United Kingdom's financial services industry, which continues a notable history of being based in the City.
The term London now refers to a much larger conurbation roughly corresponding to the London region, which is also known as the Greater London administrative area, of 32 boroughs (including the City of Westminster), in addition to the City of London. The local authority for the City, the City of London Corporation, is unique in the United Kingdom, and has some unusual responsibilities for a local authority in Britain, such as being the police authority for the City. It also has responsibilities and ownerships beyond the City's boundaries. The Corporation is headed by the Lord Mayor of the City of London, an office separate from (and much older than) the Mayor of London.