(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.