brother (Noun) — (Roman Catholic Church) a title given to a monk and used as form of address. ex. "a Benedictine Brother"
brother (Noun) — A male with the same parents as someone else. ex. "my brother still lives with our parents"
brother (Noun) — (religion) a male person who is a fellow member (of a fraternity, religion or other group). ex. "none of his brothers would betray him"
brother (Noun) — A close friend, esp. one who often acts as a companion.
brother (Noun) — Used as a term of address for those male persons engaged in the same movement. ex. "Greetings, brother!"