Definition
One who makes a promise; the party to a contract or obligation who undertakes to perform. In modern usage, the term appears as an alternative spelling of "promisor" and carries the same meaning: the party who is bound by a promise to do or refrain from doing something, as distinguished from the promisee, who receives the benefit of that promise.
In the civil law tradition, the term carries a more precise technical meaning. Within the Roman stipulatio — a formal oral contract constituted by a prescribed exchange of question and answer — the promissor was the party who responded to the stipulator's interrogation. The stipulator asked "Promittis?" ("Do you promise?") or more solemnly "Spondes?" ("Do you undertake?"), and the promissor replied "Promitto" or "Spondeo." The resulting obligation was unilateral: only the promissor was bound. This civil law sense distinguishes promissor as a term of art from its broader modern usage as a simple synonym for promisor.
Common Language
Modern common usage (Wiktionary): Alternative form of promisor.
Historical common usage (Webster's 1913): Not independently defined; subsumed under "promisor."
The gap here is not between legal and common meaning but between the civil law technical sense and the general modern sense. In ordinary and modern legal usage, promissor and promisor are interchangeable. In civil law and Roman law sources, promissor names a specific procedural role within the stipulatio, which carries implications about the form, structure, and unilateral nature of the obligation that the generic term promisor does not convey.
Common Confusion
Promissor / Promisor: These are spelling variants of the same concept in modern usage, and researchers should treat them as functionally identical in contemporary legal documents and case law. The double-s spelling reflects the Latin root and is more common in older common law and civil law texts; single-s promisor dominates modern American legal usage. Do not assume a substantive distinction based on spelling alone.
Promissor / Stipulator: In civil law sources, these are the two parties to a stipulatio and are not interchangeable. The stipulator initiates and receives the promise; the promissor undertakes it. Conflating the two reverses the direction of the obligation.
Why It Matters in Research
Researchers will encounter both spellings — promissor and promisor — across the Law Mind corpus, and the distribution is not random. The double-s form tends to cluster in older common law treatises, civil law commentary, and sources with strong Roman law influence. When tracing contract doctrine through historical sources, the spelling can serve as a rough signal of the source's intellectual tradition, though it is not a reliable one and should not be treated as such.
The more important research trap is the civil law technical sense. When Burrill, Black, or a civil law treatise uses promissor, it is often operating within the stipulatio framework, where the term has specific procedural content: the form of the exchange matters, the obligation is unilateral, and the promissor's response must mirror the stipulator's question or the contract fails. Reading this back into a common law contract context — where mutual promises, consideration, and offer-and-acceptance govern — will produce analytical error.
Rapalje & Lawrence's entry is instructive for a different reason: it pivots from promissor directly into promissory note without a hard break, and the entry on the note defines the signing party as the "maker," not the promissor. This reflects the commercial law divergence where negotiable instruments developed their own vocabulary, and the promisor/promissor terminology receded in favor of maker, drawer, and endorser. Researchers working in the commercial paper or negotiable instruments area should trace the vocabulary shift and not assume that promissor in a bills-and-notes context carries the same meaning as in contract or civil law discussions.
Historical Dictionary Support
The four source dictionaries are in close agreement on the core civil law definition, with Burrill providing the most complete account. Black's (both editions) and Burrill share nearly identical text, suggesting common derivation or direct borrowing. Burrill adds the formal exchange — the stipulator's question and the promissor's reply — and the Institutes citation (Inst. 3.16.1; 3.17 pr.), grounding the definition in Justinian's Institutes. This is the most useful formulation for researchers working in civil law, Roman law, or mixed-jurisdiction sources.
Rapalje & Lawrence diverges in focus. The entry acknowledges the basic meaning (one who makes a promise) but immediately transitions to promissory note, treating the latter as the more practically significant concept. This reflects a common law commercial orientation and signals that by the time of that dictionary's composition, the civil law technical sense of promissor had receded in American practice.
What the historical dictionaries collectively miss is the modern interchangeability of promissor and promisor in contemporary case law and statutory text. None of the source dictionaries addresses the spelling variant explicitly as such, which can leave a researcher uncertain whether the two forms carried distinct meanings in historical usage. They did not in substance, but the civil law form did carry different contextual freight.