The quality or state of being fervent; passionate and intense feeling, belief, or expression.
From the adjective 'fervent' (from Latin 'fervens') plus the English suffix '-ness', which is the most productive way to turn adjectives into abstract nouns in modern English.
While 'fervor' and 'fervency' have more poetic ring, 'ferventness' is how we'd naturally describe it in modern English—showing how the '-ness' suffix will always work to create new abstract nouns from any adjective.
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