Formerly, an officer who attended magistrates and judges to execute their orders.

Apparitor, or apparator, (Latin for "a servant of a public official", from apparere, "to attend in public") was an attendant who executed the orders of a Roman magistrate.

A messenger or officer who serves the process of an ecclesiastical court.

An official of the ecclesiastical courts, who summoned people to appear before them. (MEDIEV-L. Medieval Terms)