/ Dictionary / Index T thionic acid: Meaning and Definition of Find definitions for: thion'ic ac'id Pronunciation: [key] — Chem. Chem. any of the five acids of sulfur of the type HSO, where n is from two to six. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. thionic thionyl Related Content Daily Word Quiz: eureka Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language