noun pl. brothers broth′·ers, brethren breth′·ren

1. a man or boy as he is related to the other children of his parents: sometimes also used of animals
2. a man or boy related to one by having a parent in common; half brother
3. a stepbrother
4. a foster brother
5. a close male friend who is like a brother
6. a fellow human being
7. a male fellow member of the same race, church, profession, organization, etc. fraternity brother, soul brother
8. a lay member of a men's religious order
9. Informal any man: often used as a familiar term of address

Etymology: ME < OE brothor < IE base *bhrāter > Goth brothar, L frater, OIr brāthir, Sans bhrātar, Gr phratēr

interjection