To provide evidence as a witness, subject to an oath or affirmation, in order to establish a particular fact or set of facts.
Court rules require witnesses to testify about the facts they know that are relevant to the determination of the outcome of the case. Under the law a person may not testify until he is sworn in. This requirement is usually met by a witness swearing to speak the truth. A person who does not believe in appealing to God may affirm to the court that the testimony about to be given is the truth.
A witness may testify as to facts directly observed, which is called direct evidence; facts learned indirectly, which is called CIRCUMSTANTIAL EVIDENCE; or, in the case of an expert, an opinion the expert has formed based on facts embodied in a hypothetical question. The parties to the court proceeding are free to question a witness as to the truthfulness of the testimony or the competence of the witness.
The FIFTH AMENDMENT to the U.S. Constitution gives the defendant in a criminal trial the right not to testify, so as to avoid SELF-INCRIMINATION. In addition, the rule that a person must testify when called as a witness has several exceptions based on the existence of a special relationship between the defendant and the potential witness. Among the most important of these exceptions are confidential communications between a husband and a wife, an attorney and a client, a doctor and a patient, and a priest and penitent.
The RULES OF EVIDENCE govern what a person may testify about at a court proceeding. Though there are numerous exceptions, generally a witness may not testify about what she heard another say if that testimony is offered to prove the truth of the matter asserted. Such testimony is known as HEARSAY. For example, if the witness testifies that he heard that JOHN DOE was married and this statement is offered to prove that John Doe was married, it is hearsay and the court will strike the testimony from the record.