square kilometer vs area unit

square kilometer and area unit both are nouns.

Word	Noun	Adjective	Verb	Adverb
square kilometer	Yes	No	No	No
area unit	Yes	No	No	No

As nouns, area unit is a hypernym of square kilometer; that is, area unit is a word with a broader meaning than square kilometer:

square kilometer: a decimal multiple of the unit of surface area, the square metre, equal to 1,000,000 square meters.
area unit: a system of units used to measure areas

Other hypernyms of square kilometer include square measure.

square kilometer (noun)	area unit (noun)
a decimal multiple of the unit of surface area, the square metre, equal to 1,000,000 square meters.	a system of units used to measure areas