The quality or state of being better; superiority or the condition of having improved.
From 'better' plus the suffix '-ness,' which creates abstract nouns describing qualities or states. This is a morphologically sound English word, though rarely used.
Betterness is a word that almost never appears in modern writing, yet it's perfectly grammatical—it shows how English allows infinite word creation through suffixation, but actual usage is determined by whether communities of speakers find the word useful enough to adopt.
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