/ Dictionary / Index M main diagonal: Meaning and Definition of Find definitions for: main' diag'onal Pronunciation: [key] See under(def. 9). Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. main-de-fer main drag Related Content Daily Word Quiz: clairvoyant Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language