Hearsay is testimony from a witness under oath who is reciting an out-of-court statement that is being offered to prove the truth of the matter asserted.

The Federal Rules of Evidence prohibit introducing hearsay statements during applicable federal court proceedings, unless one of nearly thirty exemptions or exceptions applies. The Federal Rules of Evidence define hearsay as:

A statement that: (1) the declarant does not make while testifying at the current trial or hearing; and (2) a party offers in evidence to prove the truth of the matter asserted in the statement. (F.R.E. 801(c)).

The "declarant" is the person who makes the out-of-court statement. (F.R.E. 801(b)).

The Federal Rules define a "statement" as "a person’s oral assertion, written assertion, or nonverbal conduct, if the person intended it as an assertion." (F.R.E. 801(a)). The Supreme Court has further clarified that a "statement" refers to "a single declaration or remark, rather than a report or narrative." Thus, a trial court must separately analyze each individual statement, "sentence-by-sentence," rather than analyzing the narrative as whole for hearsay content or exceptions.

"The truth of the matter asserted" means the statement itself is being used as evidence to prove the substance of that statement. For example, if a witness says, "Margot told me she loved Matt" to prove that Margot did in fact love Matt, the witness's statement is hearsay. Thus, the reason a party offers a statement is central to determining whether it qualifies as excludable hearsay.

If a statement is being used to prove something other than the truth of what the statement asserts, it is not inadmissible because of the hearsay rule. A quintessential example is the U.S. Supreme Court case of Tennessee v Street (1985), in which a co-defendant's confession was properly admitted against the defendant—not for the hearsay purpose of directly proving that both men jointly committed a robbery and murder—but for the nonhearsay purpose of rebutting the defendant's claim that his own confession was elicited through the sheriff's coercive tactic of reading his co-defendant's confession to him.

In cases where a statement is being offered for a purpose other than the truth of what it asserts, trial judges have discretion to give the jury a limiting instruction, mandating the jury consider the evidence only for its intended, non-hearsay purpose.

Although the Federal Rules of Evidence govern federal proceedings only, 38 states have adopted the Uniform Rules of Evidence, which closely track the Federal Rules.