Revenge (also vengeance, retribution, or vendetta amongst others) consists primarily of retaliation against a person or group in response to a perceived wrongdoing. Although many aspects of revenge resemble or echo the concept of justice, revenge usually has a more injurious than harmonious goal. The goal of revenge usually consists of forcing the perceived wrongdoer to suffer the same pain that was originally inflicted.
Revenge is a hotly-contested ethical issue in philosophy. Some feel that, at the very least, the threat of revenge is necessary to maintain a just society. In some societies, it is believed that the injury inflicted in revenge should be greater than the original one, as a punitive measure. The Old Testament philosophy of "an eye for an eye" (cf. Exodus 21:24) tried to moderate the allowed damage, in order to avoid a vendetta or series of violent acts that could spiral out of controlinstead of 'tenfold' vengeance, there would be a simple 'equality of suffering'.