Definition - What does Citizen's Arrest mean?
Citizen's arrest refers to an arrest of a person who has likely committed some sort of a civil wrong by a person who is not an official from any law enforcing agency.
Justipedia explains Citizen's Arrest
Despite the name “citizen’s arrest,” the person who is arresting is given some sort of authority. This is a very old practice which dates back to English common law. Law enforcement officials often used to encourage normal citizens to help the state with criminals