Chain Reaction is an insanely addictive word association game consisting of a chain of seven words with each word being related to both the. Chain reaction definition, Physics. a self-sustaining reaction in which the fission of nuclei of one generation of nuclei produces The Oldest Words in English. Synonyms for chain - reaction at bookofraflash.net with free online thesaurus, antonyms, and definitions. Dictionary and Word of the Day.