Definition of "Brother" :
noun: a male with the same parents as someone else
"My brother still lives with our parents."
noun: (Roman Catholic Church) a title given to a monk and used as form of address
"A Benedictine Brother."
noun: a male person who is a fellow member (of a fraternity or religion or other group)
"None of his brothers would betray him."
noun: used as a term of address for those male persons engaged in the same movement
noun: a close friend who accompanies his buddies in their activities