The quality or state of being soft, fluffy, and resembling down feathers; softness and lightness.
Noun form created from 'downy' by adding the '-ness' suffix (Old English 'næss'), transforming the adjective into an abstract noun representing the quality itself.
We need abstract nouns to think about qualities independent of things—'downiness' lets us discuss softness as a concept, which is why philosophers and scientists created so many '-ness' words!
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