catalyst āļāļ·āļāļāļ°āđāļĢ catalyst āļāļ·āļāļāļ°āđāļĢ Chemistry A substance, usually used in small amounts relative to the reactants, that modifies and increases the rate of a reaction without being consumed in the process.One that precipitates a process or event, especially without being involved in or changed by the consequences: "A free press ... has remained ... a vital catalyst to an informed and responsible electorateâ ( Robert O'Neal).