the British government organization that deals with national security within Britain. Its official name is ‘the Security Service’. Culture MI5 is an abbreviation of Military Intelligence section 5. MI5's main activities used to involve protecting British military secrets and catching foreign spies. Since the early 1990s, it has been mainly responsible for collecting information about and dealing with terrorism (= the use of violence for political aims).MI5 was a very secret organization until the early 1990s, when more information about its activities was made known to the public.