catalyst

n. substance that accelerates a chemical reaction but remains unchanged by it; someone or something which provokes change

In chemistry and biology, catalysis is the acceleration (increase in rate) of a chemical reaction by means of a substance called a catalyst, which is itself not consumed by the overall reaction. More generally, one may at times call anything that accelerates a process, a "catalyst" (for example, a "catalyst for political change"). The word is derived from the Greek noun κατάλυσις, related to the verb καταλύειν, meaning to annul or to untie or to pick up.
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Cat·a·lyst n. A substance, usually used in small amounts relative to the reactants, that modifies and increases the rate of a reaction without being [more]Catalyst - Community and Resources

Noun
1. (chemistry) a substance that initiates or accelerates a chemical reaction without itself being affected
(synonym) accelerator
(antonym) anticatalyst
(hypernym) activator
(hyponym) biocatalyst
(derivation) catalyze, catalyse
(classification) chemistry, chemical science
2. something that causes an important event to happen; "the invasion acted as a catalyst to unite the country"
(hypernym) causal agent, cause, causal agency

A substance that changes the speed or yield of a chemical reaction without being consumed or chemically changed by the chemical reaction.