The City of London is a city, ceremonial county and local government district that contains the historic centre and the primary central business district of London. It constituted most of London from its settlement by the Romans in the 1st century AD to the Middle Ages, but the modern city named London has since grown far beyond the City of London boundary. The City is now only a tiny part of the metropolis of London, though it remains a notable part of central London. Administratively, it forms one of the 33 local authority districts of London; however, the City of London is not a London borough, a status reserved for the other 32 districts (including London's only other city, the City of Westminster). It is also a separate ceremonial county, being an enclave surrounded by Greater London, and is the smallest ceremonial county in the United Kingdom.

The City of London is widely referred to simply as the City (differentiated from the phrase "the city of London" by capitalising City) and is also colloquially known as the Square Mile, as it is 1.12 sq mi (716.80 acres; 2.90 km) in area. Both of these terms are also often used as metonyms for the United Kingdom's trading and financial services industries, which continue a notable history of being largely based in the City. The name London is now ordinarily used for a far wider area than just the City. London most often denotes the sprawling London metropolis, or the 32 London boroughs, in addition to the City of London itself. This wider usage of London is documented as far back as 1888, when the County of London was created.

The City of London is the business district right at the centre of London. It’s often referred to as the Square Mile.The City of London Corporation provides local authority services for the area. It also supports and promotes London as the world’s leading international financial and business centre.The City works in partnership with local communities to increase skills, employment and opportunities, enhancing the city’s culture, history and green spaces for all Londoners. It is headed by the Lord Mayor, an international ambassador for the City and UK financial and business services, a role separate from (and much older than) the Mayor of London.