Source : Webster's Revised Unabridged Dictionary (1913)

Millimicron \Mil"li*mi`cron\, n. [Milli- + micron.]
   The thousandish part of a micron or the millionth part of a
   millimeter; -- a unit of length used in measuring light
   waves, etc.

Source : WordNet®

millimicron
     n : a metric unit of length equal to one billionth of a meter
         [syn: {nanometer}, {nanometre}, {nm}, {micromillimeter},
         {micromillimetre}]