square millimeter vs American spelling

square millimeter and American spelling both are nouns.

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

square millimeter (noun)	American spelling (noun)
a unit of area measurement equivalent to 1 millimeter in length multiplied by 1 millimeter in width.	English orthography used in the United States (characterized by 'analyze', 'center', 'usable')