half-brother vs brother

half-brother and brother both are nouns.

Word	Noun	Adjective	Verb	Adverb
half-brother	Yes	No	No	No
brother	Yes	No	No	No

As nouns, brother is a hypernym of half-brother; that is, brother is a word with a broader meaning than half-brother:

Other hypernyms of half-brother include blood brother.

half-brother (noun)	brother (noun)
a brother who has only one parent in common with you	a male with the same parents as someone else
	a male person who is a fellow member (of a fraternity or religion or other group)
	a close friend who accompanies his buddies in their activities
	used as a term of address for those male persons engaged in the same movement