What is Decretum?

Legal Definition
In the civil law. A species of imperial constitution, being a judgment or sentence given by the emperor upon hearing of a cause, (quod imperator cognoscens decrevit.) Inst 1, 2, 6. In canon law. An ecclesiastical law, in contradistinction to a secular law, (lex.) 1 Mackeld. Civil Law, p. 81, § 93, (Kaufmaim's note.)
-- Black's Law Dictionary
Legal Definition
A decree; a law.
-- Ballentine's Law Dictionary