1911 Encyclopædia Britannica/Informer
INFORMER, in a general sense, one who communicates information. The term is applied to a person who prosecutes in any of the courts of law those who break any law or penal statute. Such a person is called a common informer when he furnishes evidence on criminal trials or prosecutes for breaches of penal laws solely for the purpose of obtaining the penalty recovered, or a share of it. An action by a common informer is termed a popular or qui tam action, because it is brought by a person qui tam pro domino rege quam pro se ipso sequitur. A suit by an informer must be brought within a year of the offence, unless a specific time is prescribed by the statute. The term informer is also used of an accomplice in crime who turns what is called “king’s evidence” (see Accomplice). In Scotland, informer is the term applied to the party who, in criminal proceedings, sets the lord advocate in motion.