Generally, a witness in a trial commits perjury when he knowingly and intentionally lies about a material issue. The precise definition of this crime varies by jurisdiction. Federal law prohibits perjury, 18 U.S.C. § 1621, as well as other false declarations before federal courts. § 1623. See Dunn v. U.S., 442 U.S. 100 (1979). See also State Civil Procedure Statutes.