/ Dictionary / Index P phototonus: Meaning and Definition of Find definitions for: pho•tot•o•nus Pronunciation: (fō-tot'n-us), [key] — n. Biol. the normal condition of sensitiveness to light in organisms or their organs. the irritability exhibited by cytoplasm when exposed to light of a certain intensity. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. photothermic phototopography Related Content Daily Word Quiz: pristine Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language