The quality or state of being floaty; the degree to which something tends to float or feels weightless.
From floaty (float + -y) + -ness (suffix creating abstract nouns from adjectives, from Old English -nes). This noun form allows speakers to refer to the abstract quality of buoyancy or the sensation of floating.
The suffix -ness is one of English's most productive tools for creating abstract nouns—we can attach it to almost any adjective to get 'the quality of being [adjective],' which is why 'floatiness' feels instantly understandable even if you've never heard it before.
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