The quality or state of being curious; the tendency to want to know or learn about things.
From curious + -ness (noun-forming suffix). The -ness suffix creates abstract nouns from adjectives, so curiousness is the abstract quality of being curious.
There's actually a difference between 'curiosity' and 'curiousness'—curiosity is the feeling you experience, but curiousness is more about the character trait or quality of being a curious person. It's more about who you are than what you feel.
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