The quality or state of being ethereal; extreme lightness, delicacy, or spirituality.
From ethereal plus -ness (suffix forming abstract nouns). The -ness suffix is one of the oldest ways to form nouns in English, used since Old English times.
English has two main suffixes for turning adjectives into quality nouns: -ity (from Latin, more formal — like 'reality') and -ness (from Germanic roots, more everyday — like 'realness'), so you can choose based on tone and formality.
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