The Collaborative International Dictionary of English v.0.48:

Millimicron \Mil"li*mi`cron\, n. [Milli- + micron.]
   The thousandth part of a micron or the millionth part of a
   millimeter; one nanometer; -- a unit of length sometimes used
   in measuring light waves, etc.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

millimicron
    n 1: a metric unit of length equal to one billionth of a meter
         [syn: nanometer, nanometre, nm, millimicron,
         micromillimeter, micromillimetre]