Cambridge Dictionaries online Cambridge Dictionaries online

The most popular online dictionary and thesaurus for learners of English


English definition of “MI5”


noun [+ sing/pl verb]     /ˌem.aɪˈfaɪv/
the official British organization that is responsible for protecting military and political secrets Spying and espionageGovernment departments and organizations
(Definition of MI5 noun from the Cambridge Advanced Learner's Dictionary & Thesaurus © Cambridge University Press)
Focus on the pronunciation of MI5

Word of the Day

best man

a male friend or relation of the bridegroom who stands with him and helps him during a marriage ceremony

Word of the Day


Read our blog about how the English language behaves.

Learn More

New Words

Find words and meanings that have just started to be used in English, and let us know what you think of them.

Learn More