The Square Mile financial district in the centre of LondonTown, covering roughly the area of the old Roman city of Londinium, and actually owned by a corporation, the Corporation of London. Often referred to as TheCity.
Quite an amusing misunderstanding. The Corporation of London is not a corporation, in the normal sense, it is in fact the local government of The City of London. The use of the word "corporation" to refer to a private business is (I think) more modern. It doesn't own the City, but it collects local taxes, empties the bins, and so on. (Local government in the UK has much less power than in more federal countries like the USA or Germany.) -- AndrewMcGuinness (sitting across the street from the Bow Bells)
The head man at the Corporation is the Lord Mayor (not to be confused with just the Mayor, who has much wider powers over all of London). It's interesting to note that most of the ancient-seeming, tourist-pleasing ceremonial surrounding the royal family over in the neighbouring City of Westminster is made up. Mostly that stuff dates back only to the late Victorian period, whereas much of the ceremonial surrounding the Lord Mayor is the genuine article, going back to the Civil War era, or even the late Middle Ages.