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.

See Examples For:

MI5

Like Golitsyn, Lyalin was also a “walk-in”: a Soviet agent who offered himself up, unbidden, to MI5.

From The Wall Street Journal ● Sep. 20, 2026

He asked MI5 to orchestrate his expulsion from London, which would enable him to return to Russia as something of a hero.

From The Wall Street Journal ● Sep. 20, 2026

Timothy Otty KC, representing MI5, announced the position during a High Court hearing in London on Friday.

From BBC ● Sep. 18, 2026

Beth then sued MI5 at the Investigatory Powers Tribunal, and in 2024 she sought a review of a ruling at the High Court.

From BBC ● Sep. 18, 2026

“Colonel, I don’t know if you’re aware of this MI5 man, Jarvis. He’s been following me around. Making threats, wild accusations. It’s just—it’s not right, sir!”

From "The Bletchley Riddle" by Ruta Sepetys and Steve Sheinkin

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