Prison gangs in the United States
A prison gang is an inmate organization that operates within a prison system, that has a corporate entity, exists into perpetuity, and whose membership is restrictive, mutually exclusive, and often requires a lifetime commitment. Political scientist David Skarbek argues the emergence of prison gangs are due to the dramatic increase in the prison population and inmate's demand for safety.
Source: Wikipedia — Prison gangs in the United States (CC BY-SA 4.0)