The British domestic and foreign counterintelligence service responsible for national internal security.
Military Intelligence, section five; a former official and present-day popular name for the counterintelligence agency of the British Government. MI5 was formed in 1909 as the Secret Service Bureau.
5th section of military intelligence.
British data gathering security service. Mainly concerned with internal affairs, as opposed to more field active MI6 (Think James Bond) which follows foreign affairs.
MI5 concern themselves with internal affairs and counter-espionage