What is a Qui tam?

Qui tam is a legal provision in the United States under the False Claims Act, 31 U.S.C. § 3729 et seq.), which allows for a private individual, or whistleblower with knowledge of past or present fraud committed against the U.S. federal government to bring suit on its behalf. Its name is an abbreviation of the Latin phrase “qui tam pro domino rege quam pro se ipso in hoc parte sequitur”, meaning “he who sues for the king as well as for himself."
http://en.wikipedia.org/wiki/Qui_tam