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.
One's sibling's spouse's sibling; either of two (or more) people whose siblings marry each other, from the perspective of the other.
One's spouse's sibling's spouse; either of two (or more) people who marry siblings, from the perspective of the other.