MI5

American

[em-ahy-fahyv] / ˈɛmˌaɪˈfaɪv /

noun

the government's security and counterespionage service that works in liaison with Scotland Yard's Special Branch.

MI5 British

abbreviation

Military Intelligence, section five; a former official and present-day popular name for the counterintelligence agency of the British Government

"Collins English Dictionary — Complete & Unabridged" 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Etymology

Origin of MI5

M(ilitary) I(ntelligence, Section) 5

Compare meaning

How does mi5 compare to similar and commonly confused words? Explore the most common comparisons:

MI5 vs. MI6

Example Sentences

Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.

By 2024, the security service MI5 had become aware of the activities of Wai and his group.

From BBC • May 7, 2026

In November, Britain's domestic intelligence agency MI5 warned that China was using spies disguised as head-hunters to recruit parliamentarians, through LinkedIn among others.

From Barron's • Mar. 27, 2026

It cites the case of Gill, and of the MI5 alert against Christine Lee, as examples of where the public have been alerted to techniques that can be used.

From BBC • Mar. 25, 2026

As well as MI5, he also worked with "another organisation within the UK Intelligence Community," the prosecution had told the jury.

From BBC • Mar. 20, 2026

The spy George Blake, during his years working for MI5, in the 1940s.

From "Spies: The Secret Showdown Between America and Russia" by Marc Favreau

Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023