Wiktionary
n. A gang at a prison who seeks to intimidate the other inmates, pressuring them to relinquish their food and other resources.
Wikipedia
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. Prison officials and others in law enforcement use the euphemism "security threat group" (or "STG"). The concept for this name is to take away the recognition and publicity that the term "gang" connotes when referring to people who have an interest in undermining the system.
Usage examples of "prison gang".
Bosch knew that meant the Aryan Nation, the white supremacist prison gang.
When a guy comes in, we've got to find out if he's a member of a street gang or a prison gang to know where to put him.
He reached down and ran his finger along a raised pink scar about four inches long, the memory of a prison gang fight he hadn't wanted to be in.
Dupree was an official of the Black Fist prison gang and suspected founder of the Black Revo-lutionary Armed Forces, and was believed to be responsible for a string of armed car robberies in upstate New York.