another name for the City of London (= the financial and banking centre of London), used especially in news reports. It is called this because the area of the City is about one square mile.
From Longman Business Dictionary
Square MileˌSquare ˈMile noun
the Square Mile journalismFINANCE another name for THE CITY (=the financial and banking centre of London). It is called this because the area of the City is about one square mile