An archaic or obsolete term possibly meaning difference or something that differentiates, or a variant spelling in older texts.
From Latin 'differens' (differing, present participle of differre). This appears to be an obsolete or dialectal form that largely disappeared from English, replaced by 'difference' and 'different.'
This word is a ghost—it barely appears in modern English, but it shows how Latin participles tried to become English nouns. The 'difference' version won out, but 'differen' shows earlier attempts at the word.
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