/ Dictionary / Index P principal quantum number: Meaning and Definition of Find definitions for: prin'cipal quan'tum num'ber Pronunciation: [key] — Physics. Physics. the nonnegative, integral quantum number that defines the stationary orbits in the Bohr model of the atom. Also called Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. principal point principal rafter 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