False Claims Act
AKA the Lincoln Law
Enacted during the US Civil War to combat fraud by companies that sold supplies to the Union Army. War profiteers sometimes shipped boxes of sawdust instead of guns, for instance, and some swindled the Union Army into purchasing the same cavalry horses several times. The law was enacted to entice whistleblowers to come forward by offering them a share of the money recovered.
The Qui Tam provision allows a private citizen to file a suit in the name of the US government, charging fraud by government contractors and other entities who receive or use government funds. The government then has the right to intervene and join the legal proceedings. The plaintiff may still proceed alone if the government declines to join.