Common informer

A common prosecutor

A person who habitually ferrets out crimes and offenses and lays information thereof before the ministers of justice, in order to set a prosecution on foot, not because of his office or any special duty in the matter, but for the sake of the share of the fine or penalty which the law allots to the informer in certain cases. Also used in a less invidious sense, as designating persons who were authorized and empowered to bring actions for penalties.

U. S. v. Stocking (D. C.) 87 Fed. 861; In re Barker, 56 VL 20.