The Collaborative International Dictionary of English v.0.48:

Joinder \Join"der\, n. [F. joindre. See Join, v. t.]
   [1913 Webster]
   1. The act of joining; a putting together; conjunction.
      [1913 Webster]

            Confirmed by mutual joinder of your hands. --Shak.
      [1913 Webster]

   2. (Law)
      (a) A joining of parties as plaintiffs or defendants in a
          suit.
      (b) Acceptance of an issue tendered in law or fact.
      (c) A joining of causes of action or defense in civil
          suits or criminal prosecutions.
          [1913 Webster]