What is Certiorari?

Legal Definition
Certiorari (/ˌsɜːrʃəˈrɛər/, /-ˈrɛəri/, or /-ˈrɑːri/), often abbreviated as cert. in the United States, is a writ seeking judicial review. It is issued by a superior court, directing an inferior court, tribunal, or other public authority to send the record of a proceeding for review.
-- Wikipedia
Legal Definition
A word from Law Latin, meaning "to be more fully informed." If an appellate court has the power to review cases at its discretion, certioari is the formal instrument by which that power gets used. A writ of certiorari orders a lower court to deliver its record in a case so that the higher court may review it. The U.S. Supreme Court uses certiorari to pick most of the cases that it hears.
The U.S. Supreme Court gives full consideration to but a small fraction of the cases it has authority to review. With many important categories of cases, the party seeking Supreme Court review does so by "petitioning" the Court to issue a "writ of certiorari." (See, e.g., 28 U.S.C. §§ 1254, 1257, 2350.) (Note: Some state appeals courts -- e.g., Ala., Ark., Colo., Conn., Fla., Ga., La., N.J. -- employ the same terminology.) If the Court decides to review one or more issues in such a case it grants "certiorari" (often abbreviated as "cert."). If the Court decides not to review the case it denies "cert."

While a decision to deny cert. lets the lower court's ruling stand, it does not constitute a decision by the Supreme Court on any of the legal issues raised by the case. Rule 10 of the Supreme Court Rules lists some of the considerations that may lead the Court to grant certiorari. But the decision to grant or deny cert. is discretionary. Under long-standing internal Court practice if four justices favor granting a petition for cert. it will be granted.

Originally, the writ of certiorari was a proceeding through which a superior court required a lower court to submit the full record of a case for review. Under the current rules and practice of the Supreme Court, however, key elements of the proceedings below are submitted along with a petition for certiorari. (See Supreme Court Rules, Rule 14.) And in some states the old terminology has been replaced. In Arizona, for example, relief formerly obtained by the writs of prohibition, mandamus and certiorari is now obtained through a "special action."

The Court's orders granting or denying cert. are issued as simple statements of actions taken, without explanation.

See, e.g. Jimenez v. Quarterman 555 U.S.__(2009).
Legal Definition
Practice. To be certified of; to be informed of. This is the name of a writ issued from a superior court directed to one of inferior jurisdiction, commanding the latter to certify and return to the former, the record in the particular case. Bac. Ab. h. t.; 4 Vin. Ab. 330; Nels. Ab. h. t.; Dane's Ab. Index, h. t.; 3 Penna. R. 24. A certiorari differs from a writ of error. There is a distinction also between a hab. corp. and a certiorari. The certiorari removes the cause; the hab. corp. only supersedes the proceedings in below. 2 Lord Ray. 1102.

2. By the common law, a supreme court has power to review the proceedings of all inferior tribunals, and to pass upon their jurisdiction and decisions on questions of law. But in general, the determination of such inferior courts on questions of fact are conclusive, and cannot be reversed on certiorari, unless some statute confers the power on such supreme court. 6 Wend. 564; 10 Pick. 358; 4 Halst. 209. When any error has occurred in the proceedings of the court below, different from the course of the common law, in any stage of the cause, either civil or criminal cases, the writ of certiorari is the only remedy to correct such error, unless some other statutory remedy has been given. 5 Binn. 27; 1 Gill & John. 196; 2 Mass. R. 245; 11 Mass. R. 466; 2 Virg. Cas. 270; 3 Halst. 123; 3 Pick. 194 4 Hayw. 100; 2 Greenl. 165; 8 Greenl. 293. A certiorari, for example, is the correct process to remove the proceedings of a court of sessions, or of county commissioners in laying out highways. 2 Binn. 250 2 Mass. 249; 7 Mass. 158; 8 Pick. 440 13 Pick. 195; 1 Overt. 131; 2 Overt. 109; 2 Pen. 1038; 8 Verm. 271 3 Ham. 383; 2 Caines, 179.

3. Sometimes the writ of certiorari is used as auxiliary process, in order to obtain a full return to some other process. When, for example, the record of an inferior court is brought before a superior court by appeal, writ of error, or other lawful mode, and there is a manifest defect, or a suggestion of diminution, a certiorari is awarded requiring a perfect transcript and all papers. 3 Dall. R. 413; 3 John. R. 23; 7 Cranch, R. 288; 2 South. R. 270, 551; 1 Blackf. R. 32; 9 Wheat. R. 526; 7 Halst. R. 85; 3 Dev. R. 117; 1 Dev. & Bat. 382; 11 Mass. 414; 2 Munf. R. 229; 2 Cowen, R. 38. Vide Bouv. Inst. Index, h. t.
-- Bouviers Law Dictionary
Legal Definition
Lat. (To be informed of, to be made certain in regard to.) The name of a writ issued by a superior court directing an inferior court to send up to the former some pending proceeding, or all the record and proceedings in a cause before verdict, with its certificate to the correctness and completeness of the record, for review or trial; or it may serve to bring on the record of a case already terminated below, if the inferior court is one not of record, or in cases where the procedure is not according to the course of the common law. State v. Sullivan (C. C.) 50 Fed. 593; Dean v. State, 63 Ala. 154; Railroad Co. v. Trust Co. (C. Ct) 78 Fed. 661; Fowler v. Lindsey, 3 Dali. 413, 1 In Ed. 658; Basnet v. Jacksonvlile, 18 Fla. 526; Walpole v. Ink, 9 Ohio, 144; People v. Livingston Co.unty, 48 Barb. (N. Y.) 234.

Originally, and in English practice, a certiorari is an original writ, issuing out of the court of chancery or the king's bench, and directed in the king's name to the judges or officers of inferior courts, commanding them to certify or to return the records or proceedings in a cause depending before them, for the purpose of a judicial review of their action. Jacob. In Massachusetts it Is defined by statute as a writ issued by the supreme judicial court to any interior tribunal, commanding it to certify and return to the supreme judicial court its records in a particular case, in order that any errors or irregularities which appear in the proceedings may be corrected. Pub. St. Mass. 1882, p. 1288.
-- Black's Law Dictionary
Legal Definition
A writ issued by a higher court to bring up the record of a lower court or judicial officer or body for review.
-- Ballentine's Law Dictionary