A unit of area (abbreviation sq mi or sq. mi. or mi2) equal to the area of a square the sides of which are each one mile long
The Square Mile is the part of London where many important financial institutions have their main offices. see also City. the Square Mile 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