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What does MI5 mean?

Looking for the meaning or definition of the word MI5? Here's what it means.

Proper noun
  1. The domestic intelligence agency of the United Kingdom, which operates in secret and whose primary function is to spy on terrorist organizations and others viewed as a threat to British national security. Formally known as the Security Service.
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Examples
It remained unclear how much involvement the spy agencies MI5 and GCHQ would have in the new set-up.
This is as if the Spycatcher affair ten years ago hadn't showed MI5 to be a nest of hard right conspirators.
Blunt, we are told, meets Chubb, his new MI5 debriefer, for the first time six years after his 1964 confession.
That was despite MI5 agents eventually providing documentary evidence to prove the men had been in Britain.
Whitehall is also said to be considering laws to allow transcripts of phone conversations bugged by MI5 to be used as evidence in court.
Few of the people in MI6 or MI5, the counterintelligence service, would divulge anything at all.

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