Dictionary.net

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.

To serve with a writ of subpoena; to command attendance in court by a legal writ, under a penalty in case of disobedience.

See subpena.

A written order commanding attendance in court as a witness, under a penalty for failure to obey.

To serve or summon with such a written order. Also, subpena.

n. [Latin] A writ commanding the attendance in court of the person on whom it is served as a witness, &c., under a penalty :-written also subpena.