millimicron

A unit of length equal to one thousandth (10⁻³) of a micrometer or one billionth (10⁻⁹) of a meter.

A unit of length (μμ) equal to 10 Ångström units, .001 microns, or .000001 millimeters: sometimes employed in microscopy or in stating wave-lengths of light.

The thousandth part of a micron or the millionth part of a millimeter; — a unit of length used in measuring light waves, etc.

A unit of length equal to one thousandth of a micron

a metric unit of length equal to one billionth of a meter