a male who has the same parents as you [↪ sister]:
I have two brothers, William and Mark.
elder/older/younger etc brother
My younger brother is a doctor.
little/kid brother (=younger brother)
I have to take my little brother to school.
My big brother (=older brother) has always looked after me.
my twin brother
2 spoken informal
a word meaning a black man, used especially by other black men
a male member of a group with the same interests, religion, profession etc as you
4RRC plural brothers or brethren
a male member of a religious group, especially a monk:
5 American EnglishSEC
a member of a fraternity (=a club of male university students)
soldiers who have fought together in a war