family unit vs couple

family unit and couple both are nouns.

Word	Noun	Adjective	Verb	Adverb
family unit	Yes	No	No	No
couple	Yes	No	Yes	No

As nouns, couple is a hyponym of family unit; that is, couple is a word with a more specific, narrower meaning than family unit:

Other hyponyms of family unit include match, mates, man and wife, marriage, married couple.

family unit (noun)	couple (noun)
primary social group; parents and children	a pair who associate with one another
	a pair of people who live together
	(physics) something joined by two equal and opposite forces that act along parallel lines
	two items of the same kind
	a small indefinite number

family unit (verb)	couple (verb)
	engage in sexual intercourse
	link together
	bring two objects, ideas, or people together
	form a pair or pairs