Witness, in law, a term used to designate either a person who gives evidence in a judicial or quasi-judicial proceeding or someone who attests to or is present at the execution of a legal instrument, such as a deed, an affidavit, or a will.
II JUDICIAL WITNESS
Every person who gives evidence in judicial proceedings has either to take a religious oath or to affirm. An affirmation is a solemn promise, to tell the truth. Not every person may be competent to testify as a witness; a person of unsound mind, for example, may not be a witness. A person convicted of a crime, however, is considered a competent witness. A minor may be permitted by the court to testify if the court is satisfied that the child understands the importance of telling the truth.
The attendance of a witness at a judicial or quasi-judicial proceeding is compelled by a witness order (otherwise known as a subpoena); only witnesses within the jurisdiction of the court may be subpoenaed. Failure to obey a subpoena is a contempt of court.
Witnesses may not be compelled to give evidence against themselves; a witness may refuse to answer any questions that might serve to incriminate him or her. A person called as a witness cannot be compelled to testify concerning a privileged communication, such as information given to a lawyer by a client, and may be excused from testifying about confidential communication, such as that from a police informer. The accused is not generally a competent witness for the prosecution. A spouse is a competent witness for the prosecution, but not compellable.
A witness may either be a witness of fact (for example, an eyewitness to an accident) or a professional witness who gives expert evidence of opinion (for example, doctors, accident reconstruction experts, or forensic scientists).
The examination of a witness is said to be an examination in chief when the witness is testifying on behalf of the party that has requested the testimony or has subpoenaed the witness; the examination is said to be a cross-examination when the witness is questioned by the lawyer for the opposing party. A witness may be asked leading questions in cross-examination, but not when giving evidence in chief.
III WITNESS TO A DOCUMENT
Witnesses are also necessary to ensure the legal validity of certain documents and ceremonies, for example, a will.