millimeter vs American spelling

millimeter and American spelling both are nouns.

Word	Noun	Adjective	Verb	Adverb
millimeter	Yes	No	No	No
American spelling	Yes	No	No	No

millimeter (noun)	American spelling (noun)
a metric unit of length equal to one thousandth of a meter	English orthography used in the United States (characterized by 'analyze', 'center', 'usable')