look up any word, like ratchet:
 
3.
(UK GOVERNMENT) British intelligence agency. Under the Home Secretary (w/in the British Cabinet), along with MI-6 and Government Communications Headquarters (GCHQ).

A few points: MI-5 can be written MI5. It differs from MI-6 (or MI6) in that MI-5 houses one group of specialized functions, and MI-6 houses another group. Both were founded in 1909 to supervise existing agencies of the Admiralty and the War Office. MI-5 tended to specialize in counterespionage on UK soil, but it also carried out espionage against trade union officials and Labour politicians in the UK.

Since the end of the Cold War it has focused on counterterrorism activities.
Contrary to some definitions elsewhere, MI-5 was not involved in Operation Ajax; that was carried out by the Secret Intelligence Service (SIS), also known as MI-6, as well as the US CIA.
by Primus Intra Pares July 18, 2010
 
1.
MI5
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.
 
2.
MI5
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
by Gumba Gumba April 13, 2004