What is Decretals?

Legal Definition
Eccles. law. The decretals are canononical epistles, written by the pope alone, or by the pope and cardinals, at the instance or suit of some one or more persons, for the ordering and determining some matter in. controversy, and have the authority of a law in themselves.

2. The decretals were published in three volumes. The first volume was collected by Raymundus Barcinius, chaplain to Gregory IX., about the year 1231, and published by him to be read in schools, and used in the ecclesiastical courts. The second volume is the work of Boniface VIII compiled about the year 1298, with additions to and alterations of the ordinances of his predecessors. The third volume is called the Clementines, because made by Clement V., and was published by him in the council of Vienna, about the year 1308. To these may be added the Extravagantes of John XXII. and other bishops of Rome, which, relatively to the others, are called Novelle Constitutiones. Ridley's View, &c. 99, 100,; 1 Fournel, Hist. des Avocats, 194-5.

3. The false decretals were forged. in the names of the early bishops of Rome, and first appeared about A. D. 845-850. The author of them is not known. They are mentioned in a letter written in the name of the council of Quiercy, by Charles the Bald, to the bishops and lords. of France. See Van Espen Fleury, Droit de Canon, by Andre.
-- Bouviers Law Dictionary
Legal Definition
In ecclesiastical law. Letters of the pope, written at the suit or instance of one or more persons, determining some point or question in ecclesiastical law, and possessing the force of law. The decretals form the second part of the body of canon law. This is also the title of the second of the two great divisions of the canon law, the first being called the "Decree," (decretum.)
-- Black's Law Dictionary
Legal Definition
Canonical opinions of the popes.
-- Ballentine's Law Dictionary