What is Apparitor?

Legal Definition
In ancient Rome, an apparitor (also spelled apparator in English, or shortened to paritor) was a civil servant whose salary was paid from the public treasury. The apparitores assisted the magistrates. There were four occupational grades (decuriae) among them. The highest of these was the scribae, the clerks or public notaries, followed by the lictores, lictors; viatores, messengers or summoners, that is, agents on official errands; and praecones, announcers or heralds.

The term has hence referred to a beadle in a university, a pursuivant or herald; particularly, in Roman Catholic canon law, which was largely inspired by Roman law. Apparitor remained an official title for an officer in ecclesiastical courts. They were designated to serve the summons, to arrest a person accused, and in ecclesiastico-civil procedure, to take possession, physically or formally, of the property in dispute, in order to secure the execution of the judge's sentence. This was done in countries where the ecclesiastical forum, in its substantial integrity, is recognized. He thus acts as constable and sheriff. His guarantee of his delivery of the summons is evidence of the knowledge of his obligation to appear, either to stand trial, to give testimony, or to do whatever else may be legally enjoined by the judge; his statement becomes the basis of a charge of contumacy against anyone refusing to obey summons.
-- Wikipedia
Legal Definition
An officer or messenger employed to serve the process of the spiritual courts in England and summon offendere. Cowell. In the civil law. An officer who waited upon a magistrate or superior officer, and executed his commands. Calvin; Cod. 12, 53-57.
-- Black's Law Dictionary
Legal Definition
-- Ballentine's Law Dictionary