Meanings
- The domestic intelligence and counter-espionage agency of the United Kingdom, which operates in secret and whose primary function is to spy on terrorist organizations and others viewed as a threat to British national security. Formally known as the Security Service.
The service aggressively maintained its position until I produced evidence proving it was untrue - including a recording of one of the calls with a senior MI5 officer.
2025 July 6, Daniel De Simone, “How MI5 piled falsehood on falsehood in court in the case of a spy who abused women”, in BBC News, retrieved 06 Jul 2025Synonyms: Five, 5, Security Service
Synonyms & antonyms
proper nounThe domestic intelligence and counter-espionage agency of the United Kingdom, which operates in secret and whose primary function is to spy on terrorist organizations and others viewed as a threat to British national security. Formally known as the Security Service.
Origin
Abbreviation of Military Intelligence, Section Five.
Full word history & related roots →Cite this entry
MI5. (n.d.). In WatkinsX Dictionary. Retrieved November 2, 2026, from https://dictionary.watkinsx.com/word/MI5
“MI5.” WatkinsX Dictionary, https://dictionary.watkinsx.com/word/MI5. Accessed November 2, 2026.
WatkinsX Dictionary, s.v. “MI5,” accessed November 2, 2026, https://dictionary.watkinsx.com/word/MI5.
Source: MI5 on Wiktionary, CC BY-SA 4.0.