Joined together for a common purpose or as a single group. It suggests working together in agreement.
Past participle of 'unite', from Latin 'ūnitus', from 'ūnus' (one). It became common in English from the late Middle Ages.
United is the finished result of 'to unite'—it’s what you are after the joining is done. Country names like 'United States' and 'United Kingdom' literally advertise that they’re collections of parts acting as one.
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