Definition
Magister equitum (Latin: "master of the horse") was a high military and administrative office in ancient Rome with two distinct phases of meaning across Roman constitutional history.
1. Under the Roman Republic: The magister equitum was the second-in-command appointed by a dictator (magister populi) to serve as his deputy. The dictator was required to appoint a master of the horse immediately upon taking office. The magister equitum commanded the Roman cavalry and acted as the dictator's executive subordinate, holding imperium — the formal power of command — delegated from the dictator. The office existed only for the duration of the dictatorship and terminated automatically when the dictator's term ended or was vacated.
2. Under the Roman Empire: The title evolved into a senior military command post within the imperial administrative structure. By the late empire, particularly under Diocletian and Constantine, the office transformed significantly. The magister equitum became one of the supreme military commanders of the Roman field army, paired with the magister peditum (master of the infantry). Together these commanders constituted the highest tier of Roman military command beneath the emperor. In some periods a single officer held the combined title magister equitum et peditum or magister utriusque militiae (master of both services).
Common Confusion
The Republican and Imperial versions of this office are often conflated in secondary literature, but they represent functionally distinct institutions separated by centuries of constitutional change. The Republican magister equitum was a subordinate emergency officer whose power derived entirely from and was bounded by the dictator's commission. The Imperial magister equitum was a permanent, high-ranking field commander within a bureaucratic military hierarchy. Researchers should identify which period a source concerns before treating references as interchangeable.
Why It Matters in Research
Researchers working with Roman legal sources, constitutional texts, or classical legal commentators will encounter this term in both Republican and Imperial contexts, and the institutional meaning shifts substantially between them. The Digest and other classical juristic sources addressing imperium, delegated authority, and military command frequently implicate the office in discussions of who could lawfully exercise what powers and under what conditions — making clear identification of the constitutional moment essential.
Primary sources from the late Republic (Livy, Cicero) use the term in its emergency-office sense. Sources from the late empire, including legal and administrative texts in the Corpus Juris Civilis and the Notitia Dignitatum, use it in its bureaucratic military-command sense. A researcher who reads a late antique legal reference through a Republican lens, or vice versa, will misread the authority structure being described.
The transformation of the office also illustrates the broader legal-historical phenomenon of Roman constitutional vocabulary persisting while the underlying institutional reality changed — a trap common across Roman administrative law and relevant to any research tracing legal continuity or rupture across the Principate and Dominate periods.
For historians of medieval European law, derivatives of this office — and the concept of a high military deputy holding delegated sovereign command — have analogues in medieval constabulary and marshal offices that drew on Roman administrative models. Researchers tracing the lineage of delegated military command in early medieval legal texts should be alert to this connection.
Historical Dictionary Support
Black's Law Dictionary defines the term concisely as "Master of the horse. A title of office under the Roman Empire." This definition is technically accurate but incomplete in two respects: it omits the Republican phase of the office entirely, and it underspecifies the office's constitutional significance in both periods. The Roman Empire reference in Black's reflects the office as it appeared in the legal texts most familiar to civilian jurists — primarily the Corpus Juris Civilis — but a researcher relying solely on Black's would miss the Republican institution that is arguably the more constitutionally significant form of the office for purposes of understanding Roman public law and the doctrine of delegated imperium.
No additional historical legal dictionary sources are available in the current corpus for this term. The entry therefore rests on Black's as the sole dictionary anchor, supplemented by the primary source record.
Jurisdictional Note
This term has no jurisdictional variation in the modern sense. It is a historical term of Roman public law. Its relevance to modern legal research is primarily comparative, historical, and foundational — appearing in contexts such as the history of delegated executive authority, military command structures, and the Roman constitutional antecedents of civil law systems.
Encyclopedia Cross-Reference
See Law Mind Encyclopedia — Roman Constitutional Law; Roman Imperial Administration; Dictator (Roman Office); Imperium