The Collaborative International Dictionary of English v.0.48:

Rejoinder \Re*join"der\ (-d?r), n. [From F. rejoindre, inf., to
   join again. See Rejoin.]
   1. An answer to a reply; or, in general, an answer or reply.
      [1913 Webster]

   2. (Law) The defendant's answer to the plaintiff's
      replication.
      [1913 Webster]

   Syn: Reply; answer; replication. See Reply.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Rejoinder \Re*join"der\, v. i.
   To make a rejoinder. [Archaic]
   [1913 Webster]

WordNet (r) 3.0 (2006):

rejoinder
    n 1: a quick reply to a question or remark (especially a witty
         or critical one); "it brought a sharp rejoinder from the
         teacher" [syn: rejoinder, retort, return, riposte,
         replication, comeback, counter]
    2: (law) a pleading made by a defendant in response to the
       plaintiff's replication

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "rejoinder":
   acknowledgment, answer, answering, antiphon, back answer,
   back talk, backchat, comeback, confutation, contraremonstrance,
   counteraccusation, counterblast, countercharge, counterclaim,
   counterreply, counterstatement, defense, echo, evasive reply,
   reaction, ready reply, rebuttal, rebutter, receipt, refutation,
   repartee, replication, reply, repost, rescript, rescription,
   respond, respondence, response, responsion, responsory, retort,
   return, reverberation, riposte, short answer, snappy comeback,
   surrebuttal, surrebutter, surrejoinder, witty reply, witty retort,
   yes-and-no answer



Bouvier's Law Dictionary, Revised 6th Ed (1856):

REJOINDER, pleadings. The name of the defendant's answer to the plaintiff's
replication.
     2. The general requisites of a rejoinder are, 1. It must be triable. 2.
It must not be double, nor will several rejoinders be allowed to the same
declaration. 3. It must be certain. 4. It must be direct and positive, and
not merely by way of recital or argumentative. 5. it must not be repugnant
or insensible. 6. It must be conformable to, and not depart from the plea.
Co. Litt. 304; 6 Com. Dig. 185 Archb. Civ. Pl. 278; U. S. Dig, Pleading,
XIII.