/ Dictionary / Index R reactant: Meaning and Definition of Find definitions for: re•ac•tant Pronunciation: (rē-ak'tunt), [key] — n. a person or thing that reacts. Also calledany substance that undergoes a chemical change in a given reaction. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: reactant (Thesaurus) reactance reaction Related Content Daily Word Quiz: panacea Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language