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.

The ex-wife of an MI5 agent has secured a "significant" settlement against the state in the first in a series of civil cases related to the double agent, Freddie Scappaticci.

From BBC • Jun. 30, 2026

My character is approached by MI5 and MI6.

From Los Angeles Times • Jun. 11, 2026

MI5 said Chinese agents have approached more than 20,000 people in the U.K. on LinkedIn to obtain sensitive information.

From The Wall Street Journal • Jun. 4, 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

Skilled interrogators from MI5 invited Fuchs to meetings over several weeks, gradually eliciting different details of his involvement in the Soviets' espionage ring.

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