Source : Webster's Revised Unabridged Dictionary (1913)

Micromillimeter \Mi`cro*mil"li*me`ter\, n. [Micro- +
   millimeter.]
   The millionth part of a meter.

Source : WordNet®

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