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