A writ of certiorari was originally the practical method by which an appellate court “seized itself” of jurisdiction, i.e., took control over a legal matter. The writ was an order to a lower court or administrative agency, to certify the matter to the higher court, i.e., turn over (certify) to the higher court the record (or an authentic copy of the record) in the case. In practice, it has come to describe the process whereby an appellate or reviewing court, where appeal is not as-of-right (i.e., automatic) decides it wants to hear the case.
In particular, in the United States, a petition to the United States Supreme Court is necessary to obtain most normal appeals. Such petitions must be then supported in conference by four of the Supreme Court’s nine justices. About 2 percent of such petitions are granted in most appeals; the most common ground being a split between the legal rulings of different courts of appeal. Since there is only one court of appeal for U.S. patent cases, the United States Court of Appeal for the Federal Circuit, the Supreme Court rarely hears patent appeals.