brethren brethren /ˈbrɛðrən/

noun

1. (plural) the lay members of a male religious order  ( brethren )
   (plural) the lay members of a male religious order
   

 brother brother /ˈbrʌðɝ/

noun

1. () A male with the same parents as someone else  ( blood brother , brother )
   a male with the same parents as someone else
    My brother still lives with our parents
   my brother still lives with our parents
   
   
2. () (religion) a male person who is a fellow member (of a fraternity, religion or other group)  ( brother )
   (religion) a male person who is a fellow member (of a fraternity, religion or other group)
    None of his brothers would betray him
   none of his brothers would betray him
   
3. Used as a term of address for those male persons engaged in the same movement  ( brother , comrade )
   used as a term of address for those male persons engaged in the same movement
    Greetings, comrade!
   Greetings, comrade!
   
4. A close friend who accompanies his buddies in their activities  ( brother , chum , pal , buddy , crony , sidekick )
   a close friend who accompanies his buddies in their activities
     Jake and Mark had been best buddies since kindergarten.
   Jake and Mark had been best buddies since kindergarten.
   
   

 Brother

1. (Roman Catholic Church) a title given to a monk and used as form of address  ( Brother )
   (Roman Catholic Church) a title given to a monk and used as form of address
    A Benedictine Brother
   a Benedictine Brother