A feud, also known in more extreme cases as a blood feud, vendetta, faida, clan war, gang war, private war, or mob war, is a long-running argument or fight, often between social groups of people, especially families or clans. Feuds begin because one party perceives itself to have been attacked, insulted, injured, or otherwise wronged by another. Intense feelings of resentment trigger an initial retribution, which causes the other party to feel greatly aggrieved and vengeful. The dispute is subsequently fuelled by a long-running cycle of retaliatory violence. This continual cycle of provocation and retaliation usually makes it extremely difficult to end the feud peacefully. Feuds can persist for generations and may result in extreme acts of violence. They can be interpreted as an extreme outgrowth of social relations based in family honor. A mob war is a time when two or more rival families begin open warfare with one another, destroying each other's businesses and assassinating family members. Mob wars are generally disastrous for all concerned, and can lead to the rise or fall of a family.

Revenge is defined as committing a harmful action against a person or group in response to a grievance, be it real or perceived. Primitive justice or retributive justice is often differentiated from more formal and refined forms of justice such as distributive justice and divine judgment.