The City of London is a district of Greater 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 sqmi) 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.

The City is today a major business and financial centre, ranking as the leading centre of global finance. Throughout the 19th century, the City served as the world's primary business centre, and continues to be a major meeting point for businesses to this day. London came top in the Worldwide Centres of Commerce Index, published in 2008. The insurance industry is focused around the eastern side of the City. The other major financial district in London is Canary Wharf, 2.5 mi to the east.

The City has a resident population of about 7,000 (2011) but over 300,000 people commute to and work there, mainly in the financial services sector. The legal profession forms a major component of the northern and western sides of the City - especially in the Temple and Chancery Lane areas where the Inns of Court are located, of which two—Inner Temple and Middle Temple—fall within the City of London boundary.