MI5 Syllabification: (MI5)Definition of MI5 (in the UK) the governmental agency responsible for dealing with internal security and counter-intelligence on British territory. Formed in 1909, the agency was officially named the Security Service in 1964, but the name MI5 remains in popular use. Origin:from Military Intelligence section 5MI5 in other Oxford dictionaries Definition of MI5 in the British & World English dictionary
