The act of lying under oath is called perjury in most jurisdictions, although it can also be referred to as forswearing or lying on oath. In legal terms, it requires an intentional act of lying after a person has taken an oath or affirmation to tell the truth. Penalties for this crime can vary by jurisdiction, but often include imprisonment.
Perjury is a crime in many jurisdictions. A person can commit perjury by lying under oath either verbally or in writing. Although the possible punishment for this crime can vary, under the laws of many countries, it is considered serious and therefore carries a possible prison sentence if a person is convicted. In reality, however, prosecutions for perjury are somewhat rare in most places.
A person may commit perjury by testifying under oath and making a false statement during that testimony. Under the rules of procedure in most legal systems, a witness must be "sworn in" prior to beginning his or her testimony. Swearing in includes the witnesses agreeing, under the penalties of perjury, to tell the complete truth when he or she testifies.
A mistake of fact or an unintentional misrepresentation of fact is not considered to be perjury. In order for a witness to commit perjury during his or her testimony in court, he or she must intentionally make a false statement of fact. In addition, in many places, telling a lie about something immaterial to the matter before the court is not considered perjury. The statement upon which a charge of perjury is based must be something that was relevant to the court's decision in the matter before the court.
A person may also be guilty of lying under oath in writing. Many legal documents include a statement attesting to the fact that the person executing the document swears, under the penalties of perjury, that the information contained in the document is true and accurate. Once a person signs the document, he or she is subject to a charge of perjury if a material fact was misrepresented or a false statement was made in the document.
Perjury is often charged as a felony. In the United States, at the federal level and in most state courts, it is punishable by a year or more in prison. In the United Kingdom, perjury is punishable by up to seven years in prison.