noun

• (law) A writ (“written order”) requiring someone to appear in court, or at a deposition or some other legal proceeding, as a witness to give testimony (a subpoena ad testificandum) or to produce evidence (subpoena duces tecum), in default of which the person may be punished.
  "Nothing vnder a Sub-pœna can dravv him to London, and vvhen he is there, hee ſtickes faſt vppon euery Obiect, caſts his eyes avvay vppon gazing, and becomes the prey of euery cut-purſe."
• (historical, law) A writ requiring a defendant to appear in court to answer a plaintiff's claim (a subpoena ad respondendum); in England and Wales, and Ireland, this writ was formerly issued by the Court of Chancery at the plaintiff's request to commence a suit, but the procedure was abolished in 1852.
  "No man to be called by Privy Seal or Subpœna to anſwer any matters but ſuch as have no remedy by the Common law, and that to appear ſo by the teſtimony of two Juſtices of either Bench, and by Indenture between them and the Plaintiff, which Plaintiff ſhall always appear in proper perſon, and find ſurety by recognizance to proſecute with effect the matters of the Bill only, and to anſwer dammages if the ſame fall out againſt the Plaintiff."
• (figurative) A motive or thing which can compel or demand something, or summon someone.
  "Your meetings cal'd the Ball, to vvhich appeare, / As to the Court of Pleaſure, all your gallants, / And Ladies thither bound by a Subpena / Of Venus, […]"

verb

• (law) To summon (someone) with a subpoena (noun sense 1) to appear in court, or at a deposition or some other legal proceeding, as a witness to give testimony or to produce evidence; to serve a subpoena on (someone).
  "Poor Willy! he would not even subpœna any of his old friends as to his general character. But even if he had, what could the Court do since he pleaded guilty?"
• (law) To require (a document or other evidence) to be produced before a court, or at a deposition or some other legal proceeding.
  "I have just recollected that you ought to have sub-pœnaᵈ in Goodwins & Thomass cases Goodwins comes on the second monday in October. You must send up for them."
• (figurative) To compel or summon (someone or something); to demand.
  "We wish to subpœna the public to our characters."