The quality or state of being curved outward on both sides; the geometric property of a biconvex shape.
From 'biconvex' + '-ity' (a suffix forming nouns of quality or condition). The noun form emerged naturally in mathematical and optical literature to discuss the degree or nature of this curved property.
The biconvexity of Earth (nearly spherical) is why ancient sailors eventually proved the world wasn't flat—a curved horizon visible from ships proved we lived on a biconvex planet, not a flat plane.
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