Definitions of brother:

• noun:   a male with the same parents as someone else

  Example: "My brother still lives with our parents"

• noun:   (Roman Catholic Church) a title given to a monk and used as form of address

  Example: "A Benedictine Brother"

• noun:   a male person who is a fellow member (of a fraternity or religion of other group)

  Example: "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