Something that counteracts or works against something else; a substance or force that opposes or neutralizes another force or substance.
From 'counteract' (to work against, from 'counter-' plus 'act') plus the suffix '-ant' (something that does an action). A chemical or functional agent that opposes an effect.
In chemistry and medicine, a 'counteractant' is something that neutralizes a poison or reverses an effect—like an antidote being a counteractant to venom, though 'counteragent' or 'antidote' are more common!
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