/ Dictionary / Index P principal series: Meaning and Definition of Find definitions for: prin'cipal se'ries Pronunciation: [key] — Math. Math. See Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. principal rafter principal sum 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