The acre is a unit of area and is used in the imperial and US customary systems with the symbol a.
It represents the area the size of one chain by one furlong (66 by 660 feet). This is exactly equal to 10 square chains, 1⁄640 of a square mile or 43,560 square feet.
It is generally used to measure the size of land.
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!