/ Dictionary / Index P photoreaction: Meaning and Definition of Find definitions for: pho•to•re•ac•tion Pronunciation: (fō"tō-rē-ak'shun), [key] — n. Chem. a chemical reaction that involves or requires light. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. photoproton photoreactivation Related Content Daily Word Quiz: pugilist Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language