activator

noun
ac·​ti·​va·​tor | \ ˈak-tə-ˌvāt-ər How to pronounce activator (audio) \

Medical Definition of activator

1 : a substance (as a chloride ion) that increases the activity of an enzyme — compare coenzyme
2 : a substance given off by developing tissue that stimulates differentiation of adjacent tissue also : a structure giving off such a stimulant