A square mile (sometimes written mi² ) is an American unit of measurement of area. It is the area inside a square that has each side equal to one statute mile (5,280 feet or 1,760 yards). This way of talking about area is often used to say how much land there is on a farm, or in a city or country, for example.
One square mile is just less than 2.6 square kilometres.
Square mile Facts for Kids. Kiddle Encyclopedia.