/ Dictionary / Index P principal point: Meaning and Definition of Find definitions for: prin'cipal point' Pronunciation: [key] — Optics. Optics. the point where a principal plane intersects the axis. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. principal plane principal quantum number Related Content Daily Word Quiz: deleterious Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language