Parol evidence rule
The parol evidence rule is a substantive common law rule in contract cases that prevents a party to a written contract from presenting extrinsic evidence that discloses an ambiguity and clarifies it or adds to the written terms of the contract that appears to be whole. The term of art parol literally means "word" and comes from Anglo-French, Anglo-Norman, or Legal French, which in turn is derived from ecclesiastical Latin parabola, which means "speech". It does not directly translate as "oral", which has a different origin in modern English, coming from the Latin oralis, which meant "mouth".
The supporting rationale for this rule is that since the contracting parties have reduced their agreement to a single and final writing, extrinsic evidence of past agreements or terms should not be considered when interpreting that writing, as the parties had decided to ultimately leave them out of the contract. In other words, one may not use evidence made prior to the written contract to contradict the writing. A common misconception is that it is a rule of evidence (like the Federal Rules of Evidence), but that is not the case.
parol evidence rule
n. a rule that oral evidence cannot be used to contradict the terms of a written contract