The act of numbering or counting things in pairs or twos.
From Latin 'di-' (two) + 'numerare' (to count, from 'numerus' meaning number) + suffix '-tion,' a learned formation likely from 17th-18th century philosophical or mathematical discourse.
This is such a specific technical term that it barely exists in modern English—it's almost like a ghost word that mathematicians and logicians might have used, but it never caught on because 'counting in pairs' or 'pairing' says the same thing far more simply.
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