The square mile is a unit of area and is an imperial unit with the symbol mi².
One square mile is equal in area to a square with edges one mile in length.
The Romans established the square mile; computed through a system of feet, paces and marches. The square mile's existence still remains in some places; for example "The Square Mile" is a term often used to describe the very centre of London in the UK (which just so happens to measure 1mi²).
The are is a unit of area and is a non-SI but metric unit with the symbol a.
It measures exactly 100m² which is the equivalent to 0.0247 acres.
The are is the principle unit for measuring the size of land across the world.