DELATOR, in Roman history, properly one who gave notice (deferre) to the treasury officials of moneys that had become due to the imperial fisc. This special meaning was extended to those who lodged information as to punishable offences, and further, to those who brought a public accusation (whether true or not) against any person (especially with the object of getting money).
Although the word delator itself, for "common informer," is confined to imperial times, the right of public accusation had long been in existence.
If the delator lost his case or refused to carry it through, he was liable to the same penalties as the accused; he was exposed to the risk of vengeance at the hands of the proscribed in the event of their return, or of their relatives; while emperors like Tiberius would have no scruples about banishing or putting out of the way those of his creatures for whom he had no further use, and who might have proved dangerous to himself.
AvKoc/avr1]s), in ancient Greece the counterpart of the Roman delator, a public informer.
The new profession of the delator must have given a stimulus to oratory.