/ Dictionary / Index M MI5: Meaning and Definition of Find definitions for: MI5 Pronunciation: (em'ī"fīv'), [key] — n. the government's security and counterespionage service that works in liaison with Scotland Yard's Special Branch. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. mi MI6 Related Content Daily Word Quiz: vertigo Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language