/ Dictionary / Index S substitution reaction: Meaning and Definition of Find definitions for: substitu'tion reac"tion Pronunciation: [key] — Chem. Chem. the replacement of an atom or group of atoms in a compound by another atom or group. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. substitution cipher substitutive Related Content Daily Word Quiz: paradox Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language