micromillimeter vs millimicron

micromillimeter and millimicron both are nouns.

Word	Noun	Adjective	Verb	Adverb
micromillimeter	Yes	No	No	No
millimicron	Yes	No	No	No

As nouns, micromillimeter and millimicron are synonyms defined as:

micromillimeter and millimicron: a metric unit of length equal to one billionth of a meter

Other synonyms of micromillimeter include micromillimetre, nanometer, nanometre, nm.

micromillimeter (noun)	millimicron (noun)
a metric unit of length equal to one billionth of a meter	a metric unit of length equal to one billionth of a meter