In mathematics, a quadric or quadric surface (quadric hypersurface in higher dimensions), is a generalization of conic sections (ellipses, parabolas, and hyperbolas).

https://en.wikipedia.org/wiki/Quadric

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"Quadric surfaces are defined by quadratic equations in two dimensional space. Spheres and cones are examples of quadrics."

"Note that if zmin > -radius or zmax < radius, the bottom or top of the sphere is open, and that if thetamax is not equal to 360 degrees, the sides are also open"

https://web.cs.wpi.edu/~matt/courses/cs563/talks/renderman/q...