An apocrisiarius, the Latinized form of apokrisiarios (Greek: ἀποκρισιάριος), sometimes Anglicized as apocrisiary, was a high diplomatic representative during Late Antiquity and the early Middle Ages. The corresponding (purist) Latin term was responsalis ("he who answers").^[1] The title was used by Byzantine ambassadors, as well as by the representatives of bishops to the secular authorities.^[2] The closest modern equivalent is a papal nuncio; the title apocrisiarius is also still employed by the Anglican Church.

An apocrisiarius was a cleric who served as the representative (also described as legate, a less precise term) of a patriarch or other bishop to the Byzantine imperial court of Constantinople. The office existed since the 5th century, but was institutionalized by law only under Emperor Justinian I (r. 527–565). Several of the more important ecclesiastical sees maintained permanent apocrisiarii in the imperial capital.^[1] The most important of these were the papal apocrisiarii (circa 452 till 743). The title was also used for the representative of a metropolitan archbishop at the court of his "territorial" patriarch in either Constantinople, Alexandria, Antioch, or Jerusalem and for secular officials carrying correspondence of the Byzantine emperor.^[3]

From the reign of Charlemagne (r. 768–814), the court of the Frankish king/emperor had clerical members styled apocrisiarii. However, they were only royal archchaplains decorated with the title of the ancient papal envoys, since they did not perform any diplomatic duties.^{[citation needed]}

In the modern Anglican Communion, representatives of the Archbishop of Canterbury to various churches are styled apocrisiarioi.^[4]

• Catholic Encyclopaedia - Apocrisiarius