family unit vs tribe

family unit and tribe both are nouns.

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

As nouns, tribe is a hypernym of family unit; that is, tribe is a word with a broader meaning than family unit:

Other hypernyms of family unit include clan, kin, kin group, kindred, kinship group.

family unit (noun)	tribe (noun)
primary social group; parents and children	a social division of (usually preliterate) people
	a federation (as of American Indians)
	group of people related by blood or marriage
	(biology) a taxonomic category between a genus and a subfamily