In Roman history, it was properly one who gave notice (deferre) to the treasury officials of monies that had become due to the imperial fiscus.
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 existed.
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 whom he had no further use for and who might have proved dangerous to himself.
The Council of Arles (314) issued a similar decree when it decided that Christians who accused falsely their brethren were to be forever excluded from communion with the faithful.
[6] See Owen J. Blum, OFM Peter Damian Letters 31–60 (Washington D.C.: The Catholic University of America, 1990), 49 ("being an informer and delator of my brother's crimes").
In the alternate reality TV series An Englishman's Castle, depicting a Nazi-occupied Britain, the word "delator" is revived in reference to informers helping the Nazi occupiers.