A chemical additive that normally reduces the time of a chemical reaction, without being affected by the reaction. In a few instances some catalysts can be used to retard a reaction.

The life of an industrial catalyst varies from 1,000 to 10,000 hours, after which it must be re-generated or replaced.

In some instances a catalyst can be destructive, as in the oxidation of iron to its oxide which is catalyzed by water vapor.