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

Informers were very common both in Greece and in Rome. Every corner of the streets was infested with persons who made it their constant business to pick up stories, and catch at every occasion to accuse persons of credit and reputation. These persons were called by the Greeks Συκοφάνται; for a more particular account of whom see Sycophant.

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 misdemeanour, and Infraction the latter prosecuting them. What tended to increase the number of these pestilent wretches 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 pursuing similar conduct.