The Collaborative International Dictionary of English v.0.48:

Plymouth Brethren \Plym"outh Breth"ren\
   The members of a religious sect which first appeared at
   Plymouth, England, about 1830. They protest against
   sectarianism, and reject all official ministry or clergy.
   Also called Brethren, Christian Brethren, Plymouthists,
   etc. The Darbyites are a division of the Brethren.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Dunker \Dun"ker\, prop. n. [G. tunken to dip.]
   One of a religious denomination whose tenets and practices
   are mainly those of the Baptists, but partly those of the
   Quakers; -- called also Tunkers, Dunkards, Dippers,
   and, by themselves, Brethren, and German Baptists, and
   they call their denomination the Church of the Brethren.
   [1913 Webster]

   Note: The denomination was founded in Germany in 1708, but
         after a few years the members emigrated to the United
         States; they were opposed to military service and
         taking legal oaths, and practiced trine immersion.
         [1913 Webster + WordNet 1.5]

   Seventh-day Dunkers, a sect which separated from the
      Dunkers and formed a community, in 1728. They keep the
      seventh day or Saturday as the Sabbath.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Brethren \Breth"ren\, n.;
   pl. of Brother.
   [1913 Webster]

   Note: This form of the plural is used, for the most part, in
         solemn address, and in speaking of religious sects or
         fraternities, or their members.
         [1913 Webster]

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):

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

Moby Thesaurus II by Grady Ward, 1.0:

60 Moby Thesaurus words for "brethren":
   assembly, aunt, auntie, blood brother, brother, bub, bubba, bud,
   buddy, churchgoers, class, congregation, country cousin, cousin,
   cousin once removed, cousin twice removed, daughter, father,
   first cousin, flock, fold, foster brother, frater, grandnephew,
   grandniece, granduncle, great-aunt, great-uncle, half brother,
   kid brother, laity, laymen, minyan, mother, nephew, niece,
   nonclerics, nonordained persons, nuncle, nunks, nunky, parish,
   parishioners, people, second cousin, seculars, sheep, sis, sissy,
   sister, sister-german, sistern, society, son, stepbrother,
   stepsister, unc, uncle, uncs, uterine brother