A "ghost" was a 21st century slang term for a person who lived in a Sanctuary District but had not integrated well into the environment – that is, gangs that preyed on the other prisoners. They were considered dangerous for their violence and hostility towards other residents.
Biddle Coleridge was the leader of the "ghosts" in Sanctuary District A in 2024. (DS9: "Past Tense, Part I", "Past Tense, Part II")