A substance or agent that causes something to become active or energized; something that activates or stimulates.
From Latin actus combined with the suffix -fier (from Latin facere meaning 'to make'), creating a noun for an agent that performs the action of activating.
In chemistry, the most famous actifier is heat—it's the universal activator that makes most chemical reactions go faster, which is why heating up a reaction is often the first tool chemists use!
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