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.
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.
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?
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
Blood brother. See under Blood.