millimicron vs micromillimeter

millimicron and micromillimeter both are nouns.

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

As nouns, millimicron and micromillimeter are synonyms defined as:

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

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

millimicron (noun)	micromillimeter (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