a male having the same parents as another person

NAmer, informal a black man

(pl brothers or brethren /ʹbredhrin/) a male who shares a common tie or interest, e.g. a fellow member of a trade union or fellow member of an all-male club or society

(pl brothers or brethren) a member of a men's religious order who is not in holy orders

a lay brother

(pl brothers or brethren) used as a title in some evangelical denominations: a fellow male member of the Christian Church