What is the abbreviation for millimeter?

The abbreviation for millimeter is "mm." This is derived from the metric system, where "milli-" is a prefix meaning one-thousandth. Thus, one millimeter is one-thousandth of a meter. The use of "mm" helps clearly identify measurements in precision tasks, especially in medical contexts where accurate measurements are critical for dosing medications, measuring wound sizes, or other applications.

The other options represent different units of measurement: "m" stands for meter, "in" is an abbreviation for inch, and "g" signifies gram. Each of these measures pertain to different systems (metric or imperial) and are used for different types of measurements, further emphasizing the importance of correctly using abbreviations in scientific and medical contexts.