The Collaborative International Dictionary of English v.0.48:

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

   Note: The millionth part of a meter is now termed a micron,
         and the word micromillimeter does not conform to the IS
         use of the prefix micro-.
         [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

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