/ Dictionary / Index S SIOP: Meaning and Definition of Find definitions for: SIOP Pronunciation: (sī'op), [key] — n. (formerly) the secret and central U.S. contingency plan for waging a nuclear war with the Soviet Union. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. Sion Siouan Related Content Daily Word Quiz: ounterfeit Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language