Noun | 1. | micromillimeter - a metric unit of length equal to one billionth of a meter metric linear unit - a linear unit of distance in metric terms A, angstrom, angstrom unit - a metric unit of length equal to one ten billionth of a meter (or 0.0001 micron); used to specify wavelengths of electromagnetic radiation micrometer, micron - a metric unit of length equal to one millionth of a meter |