qui tam

from The Century Dictionary.

• noun In law, an action on a penal statute, brought partly at the suit of the people or state and partly at that of an informer: so called from the words of the old common-law writ, “Qui tam pro domino rege quam pro se ipso,” etc.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun law A writ whereby a private individual who assists a prosecution can receive all or part of any penalty imposed.