Source : Webster's Revised Unabridged Dictionary (1913)

Brother \Broth"er\ (br[u^][th]"[~e]r), n.; pl. {Brothers}
   (br[u^][th]"[~e]rz) or {Brethren} (br[e^][th]"r[e^]n). See
   {Brethren}. [OE. brother, AS. br[=o][eth]or; akin to OS.
   brothar, D. broeder, OHG. pruodar, G. bruder, Icel.
   br[=o][eth]ir, Sw. & Dan. broder, Goth. br[=o][thorn]ar, Ir.
   brathair, W. brawd, pl. brodyr, Lith. brolis, Lett. brahlis,
   Russ. brat', Pol. & Serv. brat, OSlav. bratr[u^], L. frater,
   Skr. bhr[=a]t[.r], Zend bratar brother, Gr. fra`thr, fra`twr,
   a clansman. The common plural is {Brothers}; in the solemn
   style, {Brethren}, OE. pl. brether, bretheren, AS. dat. sing.
   br[=e][eth]er, nom. pl. br[=o][eth]or, br[=o][eth]ru.
   [root]258. Cf. {Friar}, {Fraternal}.]
   1. A male person who has the same father and mother with
      another person, or who has one of them only. In the latter
      case he is more definitely called a half brother, or
      brother of the half blood.

            Two of us in the churchyard lie, My sister and my
            brother.                              --Wordsworth.

   2. One related or closely united to another by some common
      tie or interest, as of rank, profession, membership in a
      society, toil, suffering, etc.; -- used among judges,
      clergymen, monks, physicians, lawyers, professors of
      religion, etc. ``A brother of your order.'' --Shak.

            We few, we happy few, we band of brothers, For he
            to-day that sheds his blood with me Shall be my
            brother.                              --Shak.

   3. One who, or that which, resembles another in distinctive
      qualities or traits of character.

            He also that is slothful in his work is brother to
            him that is a great waster.           --Prov. xviii.
                                                  9.

            That April morn Of this the very brother.
                                                  --Wordsworth.

   Note: In Scripture, the term brother is applied to a kinsman
         by blood more remote than a son of the same parents, as
         in the case of Abraham and Lot, Jacob and Laban. In a
         more general sense, brother or brethren is used for
         fellow-man or fellow-men.

               For of whom such massacre Make they but of their
               brethren, men of men?              --Milton.

   {Brother Jonathan}, a humorous designation for the people of
      the United States collectively. The phrase is said to have
      originated from Washington's referring to the patriotic
      Jonathan Trumbull, governor of Connecticut, as ``Brother
      Jonathan.''

   {Blood brother}. See under {Blood}.

Brother \Broth"er\, v. t. [imp. & p. p. {Brothered}.]
   To make a brother of; to call or treat as a brother; to admit
   to a brotherhood. --Sir W. Scott.

Source : WordNet®

brother
     n 1: a male with the same parents as someone else; "my brother
          still lives with our parents" [syn: {blood brother}]
          [ant: {sister}]
     2: a male person who is a fellow member (of a fraternity or
        religion of other group); "none of his brothers would
        betray him"
     3: a close friend who accompanies his buddies in their
        activities [syn: {buddy}, {chum}, {crony}, {pal}, {sidekick}]
     4: used as a term of address for those male persons engaged in
        the same movement; "Greetings, comrade!" [syn: {comrade}]
     5: (Roman Catholic Church) a title given to a monk and used as
        form of address; "a Benedictine Brother"
     [also: {brethren} (pl)]