MM
an abbreviation for one million, seen in a few traditional units such as those listed below. The abbreviation is meant to indicate one thousand thousand, M being the Roman numeral 1000. However, it actually means 2000, not one million, in Roman numeration. Since the single letter M is now used commonly for one million, the use of the double MM is confusing and strongly discouraged.

m/m
an abbreviation for "by mass," used in chemistry and pharmacology to describe the concentration of a substance in a mixture or solution. 2% m/m means that the mass of the substance is 2% of the total mass of the solution or mixture.