What is Hearsay?

Legal Definition
Hearsay evidence is "an out-of-court statement introduced to prove the truth of matter asserted therein". In certain courts, hearsay evidence is inadmissible (the "Hearsay Evidence Rule") unless an exception to the Hearsay Rule applies.

For example, to prove Tom was in town, the attorney asks a witness, "What did Susan tell you about Tom being in town?" Since the witness's answer will rely on an out-of-court statement that Susan made, Susan is not available for cross-examination, and it is to prove the truth that Tom was in town, it is hearsay. A justification for the objection is that the person who made the statement is not in court and thus is insulated from cross-examination. Note, however, that if the attorney asking the same question is not trying to prove the truth of the assertion about Tom being in town but the fact that Susan said the specific words, it may be acceptable. For example, it would be acceptable to ask a witness what Susan told them about Tom in a defamation case against Susan because now the witness is asked about the opposing party's statement that constitutes a verbal act.

The hearsay rule does not exclude the evidence if it is an operative fact. Language of commercial offer and acceptance is also admissible over a hearsay exception because the statements have independent legal significance.

Double hearsay is a hearsay statement that contains another hearsay statement itself. For example, a witness wants to testify that "a very reliable man informed me that Wools-Sampson told him". The statements of the very reliable man and Wools-Sampson are both hearsay submissions on the part of the witness, and the second hearsay (the statement of Wools-Sampson) depends on the first (the statement of the very reliable man). In a court, both layers of hearsay must be found separately admissible. In this example, the first hearsay also comes from an anonymous source, and the admissibility of an anonymous statement depends upon the discharge of an additional legal burden of proof.

Many jurisdictions that generally disallow hearsay evidence in courts permit the more widespread use of hearsay in non-judicial hearings.
-- Wikipedia
Legal Definition
Broadly, an out-of-court statement offered to prove the truth of whatever it asserts. Hearsay evidence is often inadmissible at trial. However, many exclusions and exceptions exist. Evidence meeting the broad definition may not actually be hearsay under the court's evidence rules. Even hearsay may be admitted if exceptions are met.
Overview
Generally, an out-of-court statement is hearsay if the parties care about whether or not the statement is true. For the purposes of the rule, it does not matter whether the statement was oral or written. Generally speaking, hearsay cannot be used as evidence at trial.

For example, suppose that a witness stopped at the scene of a car crash. At the crash site, an injured driver stumbled up to her and said "Martians caused the accident!" Because of the hearsay rule, the statement cannot be used as evidence that Martians caused the accident. It could, however, be used as evidence that the injured driver was capable of speech after the crash. The rule would also bar the testimony if the injured driver scribbled his message on a scrap of paper and handed it to the witness.

In general, the hearsay rule is motivated by a belief that hearsay is unreliable. There are exceptions to the rule for for particularly reliable statements, and where allowing the evidence advances public policy goals. For example, under Rule 801(d) of the Federal Rules of Evidence, a witness's prior inconsistent statements are not hearsay, due to the public policy goal of avoiding perjury. Similarly, Rule 803(6) of the Federal Rules of Evidence makes an exception to the hearsay rule for business records, because businesses have incentives to keep accurate records.

Although hearsay rules differ by jurisdiction, many state hearsay rules are modeled on the Federal Rules of Evidence, which contains about 30 exceptions and exclusions to the hearsay rule. See state civil procedure rules; Federal Rules of Evidence.

See Civil Procedure, Evidence. For more on some hearsay exceptions, see Forfeiture by wrongdoing, Present sense impression, Declaration against interest, Dying declaration, Excited utterance, Past Recollection Recorded, business record exception, and former testimony exception.
Legal Definition
A term applied to that species of testimony given by a witness who relates, not what he knows personally, but what others have told him or what he has heard suid by others. Hopt v. Utah, 110 U. S. 574, 4 Sup. Ct 202, 28 L. Ed. 262; Moreli v. Morell, 157 Ind. 179, 60 N. E. 1092; Stockton v. Williams, 1 Doug. (Mich.) 570; People v. Kraft, 91 Hun, 474, 36 N. Y. Supp. 1034. Hearsay evidence is that which does not derive its value solely from the credit of the witness, but rests mainly on the veracity and competency of other persons. The very nature of the evidence shows its weakness, and it is admitted only in specified cases from necessity. Code Ga. 1882, ยง 3770; 1 Phli. Ev. 185. Hearsay evidence is second-hand evidence, as distinguished from original evidence; it is the repetition at second-hand of what would be original evidence if given by the person who originally made the statement.
-- Black's Law Dictionary