Definition
Parol evidence has two related but distinct meanings that must be kept separate.
1. In its original and broader sense, parol evidence means oral or verbal evidence — testimony delivered by word of mouth by a witness in court, as distinguished from documentary or written evidence. This is the classical meaning preserved in historical legal dictionaries and reflects the term's roots in the French parole (word, speech).
2. In its modern and dominant sense, parol evidence refers to any extrinsic evidence — oral or written — offered to vary, contradict, add to, or explain the terms of a written contract that the parties intended as a final expression of their agreement. In this usage, "parol" no longer means exclusively oral. A prior written draft, a side letter, or a prior negotiation document can all constitute parol evidence if offered to contradict an integrated written agreement.
The second meaning is the one researchers will encounter in nearly all modern contract law contexts, typically in connection with the parol evidence rule.
Common Language
Modern common usage (Wiktionary): "Parol" in ordinary modern English is largely archaic outside legal contexts. Where used, it means "given by word of mouth; oral."
Historical common usage (Webster's 1913): "Parol" meant "a word; speech; oral declaration" and was contrasted with written communication. In military usage, a parol was a spoken password or countersign.
The gap matters: Researchers approaching the term through its ordinary English meaning will assume parol evidence means exclusively oral evidence. Modern contract law has abandoned that restriction — a written preliminary draft offered to contradict a final integrated agreement is parol evidence. Relying on the common meaning will produce fundamental misreadings of both judicial opinions and secondary sources.
Common Confusion
Parol evidence is frequently confused with two things. First, researchers conflate it with the parol evidence rule, which is a doctrine governing admissibility — the rule is the legal consequence of the evidence's status, not the evidence itself. Second, the older dictionaries' definition (oral evidence given by witnesses) can be mistaken as the complete modern definition. In historical sources predating the twentieth century, the term often appears in its broad oral-evidence sense with no connection to contract integration. Context is essential: parol evidence appearing in an evidence treatise means something different than parol evidence appearing in a contracts dispute.
Additionally, do not confuse parol evidence with parole (criminal law), which concerns conditional release from incarceration and shares only a spelling variant and etymological origin.
Why It Matters in Research
The definitional split is a genuine research trap. A researcher reading an eighteenth- or nineteenth-century case or treatise that discusses "parol evidence" may find the term used in the older sense — oral testimony by witnesses — with no connection to contract integration. The modern parol evidence rule crystallized significantly through nineteenth-century American case law and treatise writing; before that consolidation, the terminology was looser.
Jurisdictional variation in the parol evidence rule itself is significant and affects how courts characterize what counts as parol evidence in the first place. Under the Uniform Commercial Code (Article 2, section 2-202), courts may admit consistent additional terms even against a written contract unless the writing was intended as a complete and exclusive statement of agreement — a standard that differs from the common law rule applied to non-goods contracts.
The corpus contains references that cut across both meanings. Sources touching evidence law (Burrill, Bouvier, Black's in the evidence context) use the oral-evidence definition. Sources touching contract law use the extrinsic-evidence definition. Researchers should identify which frame is operative before drawing conclusions.
Bouvier's cross-reference to Browne's treatise on Parol Evidence is a useful signal: that treatise (Browne, The Parol Evidence Rule, 1882) is an early systematic treatment of the doctrine in its contract sense and helped standardize the modern usage.
Historical Dictionary Support
All four shelf sources define parol evidence primarily in its older sense — oral or verbal evidence given by witnesses in court. Black's and Burrill track nearly identical language drawn from Blackstone's Commentaries. Bouvier provides the sparest entry but usefully cross-references the American State Reports and Harvard Law Review, signaling awareness that the concept had a contested doctrinal dimension. Rapalje & Lawrence begins to gesture at the rule itself, noting that oral evidence generally cannot be substituted for written instruments, which is the earliest hint in the shelf sources of the contract-law parol evidence rule.
None of the four shelf sources fully captures the modern understanding of parol evidence as any extrinsic evidence — written or oral — offered against an integrated agreement. This is the most important gap in the historical dictionary coverage. Researchers relying solely on these definitions will miss the rule's modern scope.
Jurisdictional Note
The parol evidence rule is common law in most U.S. jurisdictions for contracts outside the UCC. Article 2 of the UCC modifies the rule for contracts for the sale of goods, permitting evidence of course of dealing, usage of trade, and course of performance regardless of integration. Some jurisdictions apply more liberal exceptions than others, particularly for fraud and mistake. The Restatement (Second) of Contracts sections 209–218 provides the most influential modern synthesis.
Encyclopedia Cross-Reference
contracts_40: Interpretation — Parol Evidence Rule (Complete vs. Partial Integration)
contracts_41: Interpretation — Exceptions to the Parol Evidence Rule (Fraud, Mistake, Condition Precedent)