The quality or condition of being capable of being demonstrated; the capacity to be proved or shown.
From demonstrate + -able (capable of) + -ity (the state or quality of). This is essentially a synonym for demonstrability, showing English's tendency to create multiple forms with slightly different emphasis.
English creates both 'demonstrability' and 'demonstratability'—both mean the same thing—which is why linguists say English is gloriously redundant and messy compared to languages with stricter rules.
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