(informator) in Law, a person that informs against, or prosecutes in any of the king's courts, those that offend against any law or penal statute. See INFORMATION.

Informers were very common both in Greece and Rome. Every corner of the streets was peopled with swarms of turbulent rascals, who made it their constant business to pick up stories and catch at every occasion to accuse persons of credit and reputation: these by the Greeks were called Συνεπάθεις; for a more particular account of whom, see the article SYCOHANT.

Amongst the Romans, informers were of two sorts, mandatores and delatores. These played into each other's hands; the former marking down such persons as they pretended to have found guilty of any misdemeanor, and the other prosecuting them. What tended to increase the number of these petition fellows was, that the informers were entitled to a fourth part of the effects of the person convicted. Wicked princes rewarded and countenanced this mischievous tribe; but Titus set on foot a most diligent search after them, and punished such as he found with death or banishment. Trajan also is praised by Pliny for a similar conduct.