What is Qui Tam?

Legal Definition
In common law, a writ of qui tam is a writ whereby a private individual who assists a prosecution can receive all or part of any penalty imposed. Its name is an abbreviation of the Latin phrase qui tam pro domino rege quam pro se ipso in hac parte sequitur, meaning "[he] who sues in this matter for the king as well as for himself."

The writ fell into disuse in England and Wales following the Common Informers Act 1951 but remains current in the United States under the False Claims Act, 31 U.S.C. § 3729 et seq., which allows a private individual, or "whistleblower," with knowledge of past or present fraud committed against the federal government to bring suit on its behalf. There are also qui tam provisions in 18 U.S.C. § 962 regarding arming vessels against friendly nations, 25 U.S.C. § 201 regarding violating Indian protection laws, 46a U.S.C. 723 regarding the removal of undersea treasure from the Florida coast to foreign nations, and 35 U.S.C. § 292 regarding false marking. In February 2011, the qui tam provision regarding false marking was held to be unconstitutional by a U.S. District Court, and in September of that year, the enactment of the Leahy-Smith America Invents Act effectively removed qui tam remedies from § 292.
-- Wikipedia
Legal Definition
In a qui tam action, a private party called a relator brings an action on the government's behalf. The government, not the relator, is considered the real plaintiff. If the government succeeds, the relator receives a share of the award. Also called a popular action.

For example, the federal False Claims Act authorizes qui tam actions against parties who have defrauded the federal government. 31 U.S.C. § 3279 et seq. If successful, a relator in a False Claims Act qui tam action may receive up to 30% of the government's award.
Illustrative caselaw
See, e.g. United States ex rel Eisenstein v. City of New York, 129 S.Ct. 2230 (2009).
See also
  • Intervention (the government may intervene in a qui tam action)
Legal Definition
Remedies. Who as well. When a statute imposes a penalty, for the doing or not doing an act, and gives that penalty in part to whosoever will sue for the same, and the other part to the commonwealth, or some charitable, literary, or other institution, and makes it recoverable by action, such actions are called qui tam actions, the plaintiff describing himself as suing as well for the commonwealth, for example, as for himself. Espin. on Pen. Act. 5, 6; 1 Vin. Ab. 197; 1 Salk. 129 n.; Bac. Ab. h. t.
-- Bouviers Law Dictionary
Legal Definition
Lat "Who as well--------------." An action brought by an ihformer, under a statute which establishes a penalty for the commission or omission of a certain act, and provides that the same shall be recovorable in a civil action, part of the penalty to go to any person who will bring such action and the remainder to the state or some other institution, is called a "qui tarn action;" because the plaintiff states that he sues as well for the state as for himself. See In re Barker, 56 Vt. 14; Grover v. Morris, 73 N. Y. 478
-- Black's Law Dictionary