(1):

(a.) Appertaining to a decree; containing a decree; as, a decretal epistle.

(2):

(a.) An authoritative order or decree; especially, a letter of the pope, determining some point or question in ecclesiastical law. The decretals form the second part of the canon law.

(3):

(a.) The collection of ecclesiastical decrees and decisions made, by order of Gregory IX., in 1234, by St. Raymond of Pennafort.