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!
The square centimetre is a unit of area and is a multiple of the square metre; an SI derived unit. It has the symbol cm².
A square centimetre is equal to the area of a square with edges 1cm long. There are 10,000 square centimetres in a square metre.
It is often used to describe areas smaller than 1m² - including much work in classrooms on paper.