In chemistry, a negatively or positively charged ion that balances the charge of another ion in a solution.
From counter- + ion, from Greek ion (going), neuter present participle of ienai (to go). The term emerged in chemistry in the 19th century to describe ions that neutralize each other's charges.
Table salt is actually made of sodium ions (positive) and chloride ions (negative) that are counterions to each other—they balance out to make something stable and totally safe to eat!
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