Dictionary.net

(Hist.) This name is specially used to denote the collection of letters and decrees of the twenty popes from Clement to Melchiades, published during the pontificate of Nicholas I., 858-867. These spurious decretals, which were certainly completed after 829, assert the papal supremacy, and contain the whole Roman system of dogma and discipline.- Milman, Hist. of Latin Christianity.

[L., a decree.] A portion of Canon law, the decrees or written answers of early popes upon disputed questions. So the Romans had regarded the responsa prudentum when unanimous, as law ; and the emperor's opinion, afterwards, when all legislative power became centred in him.