The Collaborative International Dictionary of English v.0.48:

Subpoena \Sub*poe"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.]
   [1913 Webster]

   Subpoena ad testificandum. [NL.] A writ used to procure the
      attendance of a witness for the purpose of testifying.

   Subpoena duces tecum. [NL.] A writ which requires a witness
      to attend and bring certain documents.
      [1913 Webster]

WordNet (r) 3.0 (2006):

subpoena ad testificandum
    n 1: a writ issued by court authority to compel the attendance
         of a witness at a judicial proceeding; disobedience may be
         punishable as a contempt of court [syn: subpoena,
         subpoena ad testificandum]