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]

The Collaborative International Dictionary of English v.0.48:

Subpoena \Sub*poe"na\, v. t. [imp. & p. p. Subpoenaed; p. pr.
   & vb. n. Subpoenaing.] (Law)
   To serve with a writ of subpoena; to command attendance in
   court by a legal writ, under a penalty in case of
   disobedience.
   [1913 Webster]

WordNet (r) 3.0 (2006):

subpoena
    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]
    v 1: serve or summon with a subpoena; "The witness and her
         records were subpoenaed"

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "subpoena":
   bid come, call, call away, call back, call for, call forth,
   call in, call out, call together, call up, certiorari, citation,
   cite, conjure, conjure up, convene, convoke, demand, evoke,
   garnishment, habeas corpus, indent, invoke, monition, muster,
   muster up, order up, page, preconize, recall, requisition,
   send after, send for, serve, summon, summon forth, summon up,
   summons, venire, venire de novo, venire facias, warrant, writ,
   writ of summons



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

SUBPOENA, chancery practice. A mandatory writ or process, directed to and
requiring one or more persons to appear at a time to come, and answer the
matters charged against him or them; the writ of subpoena was originally a
process in the courts of common law, to enforce the attendance of a witness
to give evidence; but this writ was used in the court of chancery for the
game purpose as a citation in the courts of civil and canon law, to compel
the appearance of a defendant, and to oblige him to answer upon oath the
allegations of the plaintiff.
     2. This writ was invented by John Waltham, bishop of Salisbury, and
chancellor to Rich. II. under the authority of the statutes of Westminster
2, and 13 Edw. I. c. 34, which enabled him to devise new writs. 1 Harr.
Prac. 154; Cruise, Dig. t. 11, c. 1, sect. 12-17. Vide Vin. Ab. h.t.; 1
Swanst. Rep. 209.



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

SUBPOENA, practice, evidence. A process to cause a witness to appear and
give testimony, commanding him to lay aside all pretences and excuses, and
appear before a court or magistrate therein named, at a time therein
mentioned, to testify for the party named, under a penalty therein
mentioned. This is usually called a subpoena ad testificandum.
     2. On proof of service of a subpoena upon the witness, and that he, is
material, an attachment way be issued against him for a contempt, if he
neglect to attend as commanded.