Chemistry . a substance that causes or accelerates a chemical reaction without itself being affected.

something that causes activity between two or more persons or forces without itself being affected.

a person or thing that precipitates an event or change

His imprisonment by the government served as the catalyst that helped transform social unrest into revolution.

a person whose talk, enthusiasm, or energy causes others to be more friendly, enthusiastic, or energetic.

noun

block , blockage , preventer , prevention

noun

adjuvant , agitator , enzyme , goad , impetus , impulse , incendiary , incentive , incitation , incitement , motivation , radical stimulus , reactant , reactionary , spark plug , spur , stimulant , synergist , wave maker , ferment , leaven , leavening , yeast , activator , spark , stimulus