/ Dictionary / Index P principal focus: Meaning and Definition of Find definitions for: prin'cipal fo'cus Pronunciation: [key] — Optics. Optics. See(def. 1). Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. principal diagonal principal ideal 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