Definition
A Latin term from Roman and canon law with two principal meanings depending on the legal system in which it appears.
1. Civil law (Roman imperial). A decretum was a species of imperial constitution — specifically, a judgment or sentence issued by the emperor himself after hearing a cause. It derived its force from the emperor's personal adjudication of a dispute rather than from general legislative pronouncement. The Institutes of Justinian define it as quod imperator cognoscens decrevit: what the emperor decreed upon cognizance of the matter. It was thus an act of imperial jurisdiction rather than legislation in the strict sense.
2. Canon law. A decretum was an ecclesiastical law or rule, issued by church authority and distinguished from secular law (lex). The term came to describe authoritative collections of such rules, most famously Gratian's Decretum (Decretum Gratiani), compiled around 1140, which organized and reconciled conflicting canons and became the foundational text of medieval canon law.
Common Confusion
Decretum is one of several Latin terms — including rescriptum, constitutio, edictum, and mandatum — used to classify the different forms of imperial pronouncement in Roman law. These are frequently conflated in secondary sources. The key distinction: a decretum arose from adjudication (the emperor deciding a case); a rescriptum arose from a petition answered in writing; an edictum was a general prospective order; and a mandatum was an administrative instruction to officials. In canon law, decretum should be distinguished from decretal (decretalis), which referred specifically to a papal letter resolving a question of canon law — the decretal had legislative force across the church, while decreta in the Gratian sense were earlier conciliar canons and patristic texts compiled for doctrinal coherence.
Why It Matters in Research
Researchers encountering decretum in historical legal sources need to determine immediately whether the context is Roman civil law or canon law — the term carries different doctrinal weight in each system and points to entirely different bodies of authority.
In civil law contexts, decreta appear in the Digest and Institutes as one of the sources of imperial law (fontes iuris). When early modern civilian jurists cite decreta, they are usually invoking the Justinianic materials, not papal or conciliar canons. Confusion is possible because the organizational terminology of the Corpus Juris Civilis and the Corpus Juris Canonici parallels and deliberately mirrors each other.
In canon law contexts, the word Decretum almost always signals Gratian's compilation specifically, and researchers should understand that Gratian's Decretum was never officially promulgated by papal authority — it was a private scholarly work that achieved authoritative status through use and reception. This matters when assessing the weight of a canon cited through Gratian versus one appearing in the later official decretal collections (Liber Extra, Liber Sextus).
The term also appears in ecclesiastical court records and early Anglo-American legal materials dealing with church discipline, probate, and matrimonial causes. In those records, decretum may describe a formal order or sentence issued by an ecclesiastical tribunal, reflecting the canon law usage. Researchers working in prerogative court records or early American church court materials should expect this narrower procedural sense.
Historical dictionaries handle the civil law meaning with reasonable consistency but tend to truncate the canon law treatment. Burrill directs researchers to Calvin's Lexicon Juridicum and Brissonius for additional senses, which is sound guidance — those compilations preserve usage that does not surface in the standard abridgments.
Historical Dictionary Support
Black's (1st and 2nd editions) and Burrill's agree closely on the core definition and share the same Institutes citation (Inst. 1.2.6) and the reference to Mackeldey's Civil Law for the canon law usage. Burrill adds the Digest citation (Dig. 1.4.1) and directs readers to Calvin and Brissonius for further senses, which is more useful guidance than Black's offers.
None of the three sources adequately treat the significance of Gratian's Decretum as a distinct and enormously influential legal text. They acknowledge the canon law meaning in passing but do not convey that "the Decretum" in medieval and early modern legal writing almost invariably refers to Gratian specifically. A researcher relying solely on these entries would miss that usage entirely.
Mackeldey (Mackeld. Civil Law, § 93, Kaufmann's note) and the supporting Roman law sources cited by Burrill remain reliable for the civil law meaning. The Institutes passage is authentic and correctly quoted.
Jurisdictional Note
Decretum has no operative role in modern American or English common law. It is primarily encountered in research involving Roman law sources, medieval and early modern canon law, civil law jurisdictions tracing their foundations to the Justinianic compilation, and ecclesiastical court records from the pre-Reformation and colonial periods.