The full form of mm is millimeter, which is a small unit of displacement (length/distance) in the metric system. The metric system is based on decimals, and there are 10mm in a centimeter and 1000mm in a meter. The base of the Greek-rooted words indicates that they are hundredths (centi) and thousandths (milli) of meters. In terms of real-world comparisons, a millimeter is roughly the size of the wire used in a standard paper clip.

It is important to note that the abbreviation "MM" has other meanings in different contexts. For example, in finance and accounting, MM denotes that the units of figures presented are in millions. In this context, MM is the same as writing "M multiplied by M," which is equal to "1,000 times 1,000," which equals 1,000,000 (one million). While Roman numerals are technically additive (MM is really 1,000 plus 1,000 or 2,000), MM is still a fairly common way of abbreviating millions, especially in certain industries like oil and gas.