What is 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 called?

decretal — (R. C. Ch.) 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. (Webster's 1913.)

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