Definition
A Latin term meaning "a single one" or "unique one," derived from the Latin *unus* (one). In legal contexts, *unicus* appears primarily in Latin maxims, doctrinal phrases, and older common law and civil law writings to denote singularity — the condition of being the only one of its kind, or a single, undivided instance of a thing, person, or obligation. The term carries no independent operative legal meaning in modern law; it functions instead as a descriptive modifier within Latin phrases and legal formulas.
Why It Matters in Research
*Unicus* is a functional rather than substantive term — it will rarely appear as a standalone legal concept but will surface as a component of Latin phrases in older treatises, civil law digests, and common law writs. Researchers encountering the word in historical sources should treat it as a qualifier modifying the noun it accompanies, not as a term of art requiring independent analysis.
The primary research value of *unicus* lies in its connection to compound Latin phrases. Burrill cross-references *unica taxatio* (a single taxation or assessment), which is the most significant legal application of the root concept. Researchers who encounter *unicus* in a primary source are almost always being directed, implicitly or explicitly, toward a phrase of that structure. Follow the phrase, not the isolated word.
The term appears with some frequency in civil law scholarship and in English equity writings that drew on Roman law sources. In those contexts, *unicus heres* (sole heir) and similar constructions carry genuine doctrinal weight — the singularity of the heir or obligor could affect the structure of succession or the divisibility of an obligation. Researchers working in ecclesiastical law, early equity, or civil law reception materials should be attentive to these compound uses.
Historical sources using Latin heavily — particularly pre-nineteenth century English treatises and early American digests modeled on civil law — will use *unicus* and its compounds without translation. Modern secondary sources will rarely use the term at all. This gap can create confusion when tracing a doctrine backward through time.
Historical Dictionary Support
Burrill's Law Dictionary provides a minimal entry: "*Unicus* — A single one. See *Unica taxatio*." This is consistent with the term's role as a building block rather than an endpoint. Burrill offers no doctrinal elaboration, which accurately reflects the term's function — it gains meaning only in combination.
Burrill's cross-reference to *unica taxatio* is the more substantive entry and should be consulted alongside this one. The brevity of the *unicus* entry is itself informative: historical legal lexicographers recognized the term as definitional infrastructure, not as a concept requiring extended treatment.
No divergence among historical sources is notable here, as the term's Latin meaning is fixed and its legal use is purely descriptive. What historical dictionaries collectively omit is any guidance on compound phrases beyond *unica taxatio* — researchers should not assume that list is exhaustive.