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catalyst

noun

A substance capable of initiating or speeding up a chemical reaction.

Chemical reaction can proceed spontaneously even without a catalyst but it would be too slow. The presence of a catalyst can make chemical reactions proceed faster by a factor of several million times. The catalyst may be chemically transformed but only transiently during the reaction. By the end of the reaction, the catalyst is regenerated unchanged, and unconsumed in the reaction. An example of catalyst is an enzyme used by biological reactions.