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 circular mil is a unit designed specifically to measure the area of a wire with a circular cross section. It has the symbol cmil.
An inch is 1000 mils. This makes the circular mil particularly useful for measuring cross sections of wire as there is no need to use the symbol π so the process is simplified.
A circular mil is a very small unit so not advised to use for large areas!