* Are units of measurement: Millimeters measure length or distance, while milligrams measure mass.

* Use the prefix "milli-": This prefix means "one thousandth" (1/1000). So, a millimeter is 1/1000 of a meter, and a milligram is 1/1000 of a gram.

However, they are different in what they measure:

* Millimeter: Measures length or distance.

* Milligram: Measures mass or weight.