A person or thing that counteracts something; an agent or force that works against or opposes something else.
From 'counteract' plus the agent suffix '-er' (one who does something), creating a noun for whatever or whoever performs the counteracting action.
While less common than 'counteracting force' or 'antidote', 'counteracter' is useful in describing people who actively resist or oppose a system—like whistleblowers being counteracters to corruption!
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