catalyst

a substance that accelerates the rate of a chemical reaction without itself changing; a person or thing that causes change [Princeton Review]

substance that speeds up a chemical reaction without itself changing [Barron's]

something that causes activity between two or more persons or forces without itself being affected [Webster's]

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 [Webster's]

a person whose talk, enthusiasm, or energy causes others to be more friendly, enthusiastic, or energetic [Webster's]