A brother by blood: a brother-german or half-brother, as distinguished from step-brothers.
An unrelated male considered to share such a strong relationship generally.
An unrelated male considered to share a brother-like relationship established by a ceremonial sharing of blood in some cultures.
A sibling sharing a single parent, as distinguished from a full sibling or sibling-german, from a step-sibling, or from a sibling-like figure such as a blood sibling.