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