A writ of certiorari is issued when the Supreme Court agrees to review a case.
Step-by-step explanation:
The Supreme Court of the US has 9 justices. If someone is not satisfied with the decision of a lower court they can petition the Supreme Court to review the case. If the case is of national importance the Supreme Court decides to take it and issues a writ of certiorari.
The writ of certiorari is a request that the Supreme Court makes to the lower courts to send the records of the case for review. Only a small percentage of cases are taken up for review by the Supreme Court.