1. MCM is an abbreviation for Thousands of Circular Mils. Electrical wire larger than 0000 (4/0) is measured in MCM, according to the cross section of the conductor. A mil is 1/1000th of an inch. MCM is an obsolete unit of measure that has been replaced by kcmil, which means the same thing as MCM. 2. When used in conjunction with the Source Four PAR MCM, is an acronym for Metal Cold Mirror, a glass reflector with a dichroic coating. As opposed to a Source Four PAR EA.