The quality of being defeasible; the state of being capable of being nullified or voided under certain conditions.
From defeasible + -ness (noun of quality). A rare noun form that emphasizes the characteristic of defeasibility.
This word is almost never used because 'defeasibility' already serves the same purpose—it's a reminder that English sometimes creates multiple ways to say the same thing, and we gradually abandon the clunky options. The victory of 'defeasibility' over 'defeasibleness' is a small example of linguistic natural selection.
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