MI5
BRITISH DOMESTIC SECURITY AGENCY
Mi5; British Security Service; Security Service (United Kingdom of Great Britain and Northern Ireland); Military Intelligence Section 5; UK Security Service; United Kingdom Security Service; Military Intelligence, Section 5; London Reception Centre; MI Five; The Security Service; MI 5; MI5 officer; SyS; British Counterintelligence Directorate; Security Service (MI5); Military Intelligence (MI5); Security Service (United Kingdom); Government Communications Planning Directorate
¦ abbreviation (in the UK) the governmental agency responsible for internal security and counter-intelligence on British territory (now officially named the Security Service).
Origin
from Military Intelligence section 5.