The square metre is a unit of area and is an SI derived unit with the symbol m².
It was introduced in 1954 when the International System of Units was established and was derived directly from one of the original 6 base units - the metre (m).
It can be prefixed with any of the SI symbols that multiply it up or down such as km² (square kilometre) or µm2 (square micrometre). It is the most commonly used unit of area and is recognised all over the world.
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).