/ Dictionary / Index P periodic acid: Meaning and Definition of Find definitions for: per'i•od'ic ac'id Pronunciation: (pûr'ī-od'ik, pûr"-), [key] — Chem. Chem. any of a series of acids derived from IO by the addition of water molecules, as HIO or HIO. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. periodic periodical Related Content Daily Word Quiz: belligerent Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language