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 km2) 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 local authority for the City, namely the City of London Corporation, is unique in the UK and has some unusual responsibilities for a local council, such as being the police authority. It is also unusual in having responsibilities and ownerships beyond its 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 Lord Mayor, as of November 2019, is William Russell. The City is made up of 25 wards, with administration at the historic Guildhall. Other historic sites include, St Paul's Cathedral, Royal Exchange, Mansion House, Old Bailey, and Smithfield Market. Although not within the City, the adjacent Tower of London is part of its old defensive perimeter. Bridges under the jurisdiction of the City include London Bridge and Blackfriars Bridge.

The City is a major business and financial centre.Throughout the 19th century, the City was the world's primary business centre, and it continues to be a major meeting point for businesses. 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, around Lloyd's building. A secondary financial district exists outside the City, at Canary Wharf, 2.5 miles (4 km) to the east.

The City has a resident population of 9,401 (ONS estimate, mid-2016) but over 500,000 are employed there, and some estimates put the number of workers in the city to be over 1 million. About three-quarters of the jobs in the City of London are in the financial, professional, and associated business services sectors. 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.