millimicron vs micromillimetre

millimicron and micromillimetre both are nouns.

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

As nouns, millimicron and micromillimetre are synonyms defined as:

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

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

millimicron (noun)	micromillimetre (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