Warrant, in law, a written order issued by a judge or magistrate that directs a police officer (or in some instances a citizen) to perform a certain act that would otherwise be illegal. Warrants are issued in both criminal and civil cases as a result of an indictment or upon a complaint or affidavit sworn by an injured party. The most common warrant is one directing the arrest of a person accused of a crime. Other warrants include those directing search of a building, seizure of property, or extradition. A warrant is not necessary for arrest when a police officer has probable cause to believe a crime has been committed.