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...