Definition
A Latin term from the civil law meaning one related by marriage, as opposed to one related by blood. An affinis is a person connected to another through the marriage of a third party — a spouse's relatives, or the relatives of one's relatives by marriage. The term also carried the secondary meaning of bordering or adjacent, reflecting its root sense of proximity (from ad, "near," and finis, "boundary" or "border"), but the dominant legal usage is relational: a person standing in a relationship of affinity to another.
The term appears most frequently in civil law discussions of degrees of relationship, where the law must distinguish consanguinei (blood relatives) from affines (relatives by marriage) for purposes of inheritance, prohibited marriages, and testimonial competency.
There is no precise single English equivalent. Burrill notes that "affined" is sometimes used in translation, but this usage is uncommon and unlikely to appear in modern legal writing without explanation.
The maxim preserved in Burrill's entry — Affinis mei affinis non est mihi affinis — captures a critical limitation: the relative-by-marriage of my relative-by-marriage is not my relative by marriage. This principle bounded the legal category carefully, preventing affinity from extending indefinitely through chains of marriage.
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Common Language
Modern common usage (Wiktionary): "Affinis" has no standard entry in modern English dictionaries; it survives as a Latin term or species-name suffix in scientific nomenclature, meaning "related to" or "resembling."
Historical common usage (Webster's 1913): Not entered as an English word. "Affinity" appears as the English derivative, with general meaning of relationship, kinship by marriage, or natural attraction.
The gap between common and legal meaning is structural rather than semantic. Modern readers encountering "affinity" will understand a general sense of connection or attraction. "Affinis" as a civil law term of art is considerably narrower: it identifies a specific legal category of persons whose relationship to another is defined solely through marriage, with calculable degrees and defined legal consequences. The casual sense of "affinity" as mere similarity or attraction has no purchase in the legal usage of affinis.
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Common Confusion
Affinis is sometimes used loosely as a synonym for cognatus (blood relative) in older translations, but the civil law drew a firm line between consanguinity and affinity. Cognates share blood; affines share a marriage link. Confusing the two affects analysis of inheritance rules, prohibited degrees of marriage, and competency questions wherever the civil law tradition governs or informs the applicable rule. Researchers should also distinguish affinis from socer, gener, nurus, and similar terms that identify specific affinal relationships by role rather than by the general category.
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Why It Matters in Research
Researchers encounter affinis primarily in three contexts: (1) civil law treatises and their common law translations, (2) ecclesiastical and canon law discussions of prohibited marriages, and (3) inheritance and succession analyses in jurisdictions with civil law foundations.
The navigational challenge is that affinis appears in Latin-heavy source material — Roman law digests, early civil law commentaries, ecclesiastical records — where it may not be indexed under an English heading. Burrill's entry is the primary English-language bridge, and its note to Mackeldey's Civil Law (Kaufman translation) points to the most accessible civil law treatment in the American legal library tradition.
The maxim Affinis mei affinis non est mihi affinis is practically important: courts and commentators applying civil law principles of prohibited degrees or inheritance standing had to determine whether a claimed relationship actually qualified as affinity in the legal sense. The maxim forecloses the chain-extension argument.
Jurisdictional trap: In Louisiana, Quebec, and other mixed civil law jurisdictions, the category of affines retains substantive significance in succession law and in certain family law provisions. Researchers working in those jurisdictions should not assume that common law rules about in-laws map cleanly onto the civil law treatment of affines.
In historical common law sources, the concept is present but the Latin term may be replaced by phrases such as "kindred by marriage," "relations by affinity," or simply "in-laws" — none of which carry the technical precision of the civil law term.
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Historical Dictionary Support
Burrill's Law Dictionary is the principal English-language source for this term, and its treatment is compact but reliable. Burrill correctly identifies the dual meaning (geographic adjacency and relational affinity), flags the absence of a direct English equivalent, and preserves the maxim. The reference to Mackeldey's Civil Law through the Kaufman translation is the standard mid-nineteenth-century American path to the Roman law sources on this point.
What Burrill does not provide — and what no period English-language legal dictionary fully provides — is a worked analysis of the degrees of affinity or the specific legal consequences attached to the affinis relationship in different civil law contexts. For that, researchers must go to the civil law treatise tradition directly: Mackeldey, Pothier, or ultimately the Corpus Juris Civilis.
No significant divergence appears among historical sources on the core meaning. The term is stable across the civil law tradition.
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Jurisdictional Note
The term and its underlying concept are most operative in civil law jurisdictions (Louisiana, Quebec, and their analogues) and in historical ecclesiastical law. Common law jurisdictions address the same underlying relationships through statutory definitions of "affinity" and "relation by marriage," which may or may not track the civil law category precisely. Researchers should verify whether the applicable jurisdiction uses civil law or common law methods to calculate degrees of affinity, as the counting methods differ.
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