micromillimetre vs micromillimeter

micromillimetre and micromillimeter both are nouns.

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

As nouns, micromillimetre and micromillimeter are synonyms defined as:

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

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

micromillimetre (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