Definition of MI5 in English:

MI5

proper noun

  • (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 5.

Pronunciation:

MI5

/ˌemˌīˈfīv/