noun

• One who, or that which, activates.
• (chemistry) Something that activates a catalyst
• (biology) Any chemical or agent which regulates one or more genes by increasing the rate of transcription.
• (biology) A type of effector that increases the rate of enzyme-mediated reactions.
• A type of dopant used in phosphors and scintillators.
• (mining) An agent that enables the flotation of a mineral or minerals.
• (chiropractic) A hand-held device that delivers a controlled fast thrust to a joint, muscle, etc., to help stimulate nervous system activity.