A feud is a prolonged state of hostilities between two families, clans, or other collective groups. Throughout history, there have been some notable feuds, including the one which ranged between the Hatfields and the McCoys in the American South, the Campbell and McDonal feud in Scotland, and the English Wars of the Roses, fought between clans competing for power. Feuds also appear in fiction, perhaps more famously in Shakespeare's play Romeo and Juliet.
In order to be considered a feud, several conditions must be met. The first is that the feud typically takes place between groups who are connected in some way. Classically, a feud has involved families or people who have allied in a clan, although a feud can also take place between companies, sports teams, or even schools. Each group has a very cohesive identity, so an attack on one member is treated like an attack on the entire group.
A feud must also be prolonged, typically continuing through several generations. Over time, the root cause of the feud may be forgotten or distorted, but the feud is carried on through a series of attacks and counterattacks. It can be very difficult to put a stop to a feud, since the collective identity of the involved groups leads people to participate in acts of vengeance in solidarity, and neither side wants to admit weakness by putting a stop to the competition.
The word was first used in the sense of a vendetta in 1425, and it is derived from the Old High German fehida, which means “contention.” Obviously, humans have been feuding with each other for much longer than this, of course. Historically, many feuds took the form of blood feuds, which involved, as one might imagine, a fair amount of physical violence up to and including murder.
A feud typically begins with a perceived insult or slight. In some cases, the perception may be legitimate, as was the case historically in nations with weak legal systems, where murderers sometimes went unpunished. A feud might be sparked by the murder of a family member, which required retaliation from the head of the family. Of course, once a family retaliated for a murder, the family which committed the crime in the first place would be obligated to retaliate in turn, setting up a vicious cycle.
Feuds are relatively unknown in most nations with strong legal systems, as people can use the law to ensure that justice is meted out for things like stolen property, murder, rape, and other violations of a family or clan. However, feuds certainly do still happen, especially among subcultures like gangs and in countries with weak or corrupt legal systems.