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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.

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

He had a fake ID card saying he was an MI5 "STRAP officer".

From BBC • Mar. 20, 2026

"Prompted in part by Beth's case, MI5 has embarked on a programme of work to reinforce the highest standards of record keeping and information management."

From BBC • Mar. 17, 2026

MI5 is currently under investigation after the BBC revealed that the security service gave false evidence to three courts while defending its handling of the agent.

From BBC • Mar. 17, 2026

Why is an MI5 agent loitering around the inn?

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