What is Apocrisarius?

Legal Definition
In ecclesiastical law. One who answers for another. An officer whose duty was to carry to the emperor messages relating to ecclesiastical matters, and to take back his answer to the petitioners. An officer who gave advice on questions of ecclesiastical law. An ambassador or legate of a pope or bishop. Spelman. See Apocrisarius cancellarius.
-- Black's Law Dictionary
Legal Definition
A messenger; an ambassador.
-- Ballentine's Law Dictionary