the former name (still used, though not officially) of the British government department responsible for state security within Britain. MI5 is short for Military Intelligence section 5. Its 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.
MI5 was a very secret organization until the early 1990s, when more information about its activities was made known to the public. Its official name is the Security Service.