Rational Surface
Rational surfaces include spheres, cylinder sides, cones. These surfaces are determined by their center and radius rather than a polynomial representation.
Part of Rhinoceros for OSX , Rhinoceros
Part Type Entity
Screenshot Rational Surfaces.jpg

Relationship to NURBS Representation

Rational surfaces are distinct from NURBS Surfaces (and NURBS geometry in general), in that they do not rely on an interpolation to evaluate, but rather an explicit formula. They may, however, be converted to a NURBS representation in many software packages - this translation is sometimes performed automatically, and without alerting the user to the change.

A Rational Sphere Converted to a NURBS Surface