Wiktionary
n. The non-SI unit for measuring the cross-sectional area of an electrical conductor: 1 CM= area of a circle having diameter of 0.001 inch; use capitals to distinguish from centimeter; 0.000 506 7 sq millimeters.
Wikipedia
A circular mil is a unit of area, equal to the area of a circle with a diameter of one mil (one thousandth of an inch). It corresponds to 5.067×10 mm². It is a unit intended for referring to the area of a wire with a circular cross section. As the area in circular mils can be calculated without reference to π, the unit makes conversion between cross section and diameter of a wire considerably easier.
The area in circular mils, A, of a circle with a diameter of d mils, is given by the formula:
A = d
In Canada and the United States the National Electrical Code (NEC) uses the circular mil to define wire sizes larger than 0000 AWG. In many NEC publications and uses, large wires may be expressed in thousands of circular mils, which is abbreviated in two different ways: MCM or kcmil. For example, one common wire size used in the NEC has a cross-section of 250,000 circular mils, written as 250 kcmil or 250 MCM, which is the first size larger than 0000 AWG used within the NEC.
Usage examples of "circular mil".
It was the Unit, not Karen, whose impenetrable shield held stubbornly every circular mil of advantage gained in making such ingress.