What is Subpoena Ad Testificandum?

Legal Definition
A subpoena ad testificandum is a court summons to appear and give oral testimony for use at a hearing or trial. The use of a writ for purposes of compelling testimony originated in the Ecclesiastical Courts of the High Middle Ages, especially in England. The use of the subpoena writ was gradually adopted over time by other courts in England and the European Continent.
-- Wikipedia
Legal Definition
Subpoena to testify. The common subpoena requiring the attendance of a witness on a trial, inquisition, or examination. 3 Bl. Comm. 369; In re Strauss, 30 App. Div. 610, 52 1ST. Y. Supp. 392,
-- Black's Law Dictionary
Legal Definition
A subpoena for one to give his testimony.
-- Ballentine's Law Dictionary