The Collaborative International Dictionary of English v.0.48:

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. dative
   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.

   Note: A brother having the same mother but different fathers
         is called a uterine brother, and one having the same
         father but a different mother is called an agnate
         brother, or in (Law) a consanguine brother. A
         brother having the same father and mother is called a
         brother-german or full brother. The same modifying
         terms are applied to sister or sibling.
         [1913 Webster +PJC]

               Two of us in the churchyard lie,
               My sister and my brother.          --Wordsworth.
         [1913 Webster]

   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.
      [1913 Webster]

            We few, we happy few, we band of brothers,
            For he to-day that sheds his blood with me
            Shall be my brother.                  --Shak.
      [1913 Webster]

   3. One who, or that which, resembles another in distinctive
      qualities or traits of character.
      [1913 Webster]

            He also that is slothful in his work is brother to
            him that is a great waster.           --Prov. xviii.
                                                  9.
      [1913 Webster]

            That April morn
            Of this the very brother.             --Wordsworth.
      [1913 Webster]

   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.
         [1913 Webster]

               For of whom such massacre
               Make they but of their brethren, men of men?
                                                  --Milton.
         [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

half brother
    n 1: a brother who has only one parent in common with you [syn:
         stepbrother, half-brother, half brother]