square kilometer vs square kilometre

square kilometer and square kilometre both are nouns.

Word	Noun	Adjective	Verb	Adverb
square kilometer	Yes	No	No	No
square kilometre	Yes	No	No	No

As nouns, square kilometer and square kilometre are synonyms defined as:

square kilometer and square kilometre: a decimal multiple of the unit of surface area, the square metre, equal to 1,000,000 square meters.

square kilometer (noun)	square kilometre (noun)
a decimal multiple of the unit of surface area, the square metre, equal to 1,000,000 square meters.	a decimal multiple of the unit of surface area, the square metre, equal to 1,000,000 square meters.