Source : Webster's Revised Unabridged Dictionary (1913)

Subpoena \Sub*p[oe]"na\, n. [NL., fr. L. sub under + poena
   punishment. See {Pain}.] (Law)
   A writ commanding the attendance in court, as a witness, of
   the person on whom it is served, under a penalty; the process
   by which a defendant in equity is commanded to appear and
   answer the plaintiff's bill. [Written also {subpena}.]

   {Subp[oe]na ad testificandum}. [NL.] A writ used to procure
      the attendance of a witness for the purpose of testifying.
      

   {Subp[oe]na duces tecum}. [NL.] A writ which requires a
      witness to attend and bring certain documents.

Source : WordNet®

subpoena duces tecum
     n : a writ issued by a court at the request of one of the
         parties to a suit; it requires a witness to bring to
         court or to a deposition any relevant documents under the
         witness's control