millimicron

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A unit of length equal to one thousandth (10^-3) of a micrometer or one billionth (10^-9) of a meter.

Century Dictionary and Cyclopedia

1. n. 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.

Wiktionary

1. n. A unit of length equal to one thousandth of a micron

GNU Webster's 1913

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

WordNet 3.0

1. n. a metric unit of length equal to one billionth of a meter