Akademik

MI5
/em"uy'fuyv"/, n. (in Great Britain)
the government's security and counterespionage service that works in liaison with Scotland Yard's Special Branch.
[M(ilitary) I(ntelligence, Section) 5]

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MI5 noun
Security Service
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Main Entry: ↑MI

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MI5 UK [ˌem aɪ ˈfaɪv] US
the British government organization that works secretly to stop other countries getting important military and political information about the UK http://www.macmillandictionary.com/med2cd/weblinks/mi5.htm
Thesaurus: uk and commonwealth government departments and organizationshyponym

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(in the UK) the governmental agency responsible for dealing with internal security and counter-intelligence on British territory. Formed in 1909, the agency was officially named the Security Service in 1964, but the name MI5 remains in popular use
Origin:
from Military Intelligence section 5

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MI5 7 [ˌem aɪ ˈfaɪv] [ˌem aɪ ˈfaɪv] noun uncountable
the British government organization that deals with national security within Britain. Its official name is ‘the Security Service’.
 
Word Origin:
[MI5] from Military Intelligence section 5.  
Culture:
MI5‘s main activities used to involve protecting British military secrets and catching foreign spies. Since the early 1990s, it has been mainly responsible for collecting information about and dealing with terrorism.
MI5 was a very secret organization until the early 1990s, when more information about its activities was made known to the public.

Useful english dictionary. 2012.