Decretal a. : Appertaining to a decree; containing a decree; as, a decretal epistle.

Decretal 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.

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

Example:
Why do you think the "False or Forged Decretals" are so hard to find for study?

Taxonomy
decretals, canonical collections, Decretals of Gregory IX, papal decretals, Decretum, pontifical, pope, Extravagantes communes, Corpus Juris Canonici, Decretal, the popes, Gregory IX, compilation, Corpus Juris, canons, legislation, John, Decree of Gratian.

Defination from the web:
Decretal collections. 3 The Quinque Compilationes Antiquae Decretalium. 4 The Decretals of Gregory IX. 5 Later Collections. 6 ... It is generally stated that the most ancient decretal is the letter of Pope Saint Siricius (384-398) to Himerius, Bishop of Tarragona in Spain, ...