A learned treatise, in the law of evidence, is a text that is sufficiently authoritative in its field to be admissible as evidence in a court in support of the contentions made therein.
Under the common law, such evidence was at one time considered hearsay - a statement made out of court being introduced to prove the truth of the statement - and was not admissible "except" to rebut the testimony of an opposing expert witness. There were four ways to introduce such evidence:
Under the Federal Rules of Evidence 803 (18), either party can introduce a learned treatise as evidence, irrespective of whether it is being used to rebut the opposing party. Such texts are now considered an exception to hearsay, with two limitations:
References.
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