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, [[USC:18:1621#content|18 U.S.C. § 1621]], as well as other false declarations before [[wex:federal courts]]. [[USC:18:1623#content|§ 1623]]. See Dunn v. U.S., [[USR:442:100|442 U.S. 100]] (1979). See also [[wex:table_civil_procedure|State Civil Procedure Statutes]].