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]

The Collaborative International Dictionary of English v.0.48:

micron \mic"ron\, n. [NL., fr. Gr. mikro`s small.] (Physics)
   A measure of length; the thousandth part of one millimeter;
   the millionth part of a meter.
   [1913 Webster]

WordNet (r) 3.0 (2006):

micron
    n 1: a metric unit of length equal to one millionth of a meter
         [syn: micron, micrometer]

The Free On-line Dictionary of Computing (30 December 2018):

micrometre
micron

    (Or "micron") One millionth of a metre.  The symbol
   is a Greek letter mu followed by "m".

   Features on modern integrated circuits are typically
   measured in microns.  The smallest features in 1999 are around
   0.1 microns across.

   (1999-09-28)


The Free On-line Dictionary of Computing (30 December 2018):

Micron Electronics, Inc.
Micron

    An electronics company that develops, markets,
   manufactures, and supports high-performance notebook
   computers, desktop personal computers, PC servers and
   related hardware and software products.

   (1997-03-10)