A person or thing that confirms; someone or something that provides verification or establishes the truth of something.
From 'confirm' (Latin 'confirmare') plus the agent suffix '-er' (one who does the action). The '-er' suffix comes from Old English and Germanic roots and is the most productive agent noun suffix in English.
In blockchain technology, 'confirmers' (nodes that validate transactions) have revived medieval legal terminology where witnesses would 'confirm' someone's claim—technology often recycles old linguistic structures for new purposes.
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