NURBS Surface
A NURBS surface is like a rectangular stretchy rubber sheet. The NURBS form can represent simple shapes, such as planes and cylinders, as well as free-form, sculptured surfaces. All NURBS surfaces have an inherently rectangular organization.
Part of Rhinoceros , Rhinoceros for OSX
Part Type Entity
Screenshot Rhino NURBS Surface.jpg

Non-Uniform Rational B-Splines, are mathematical representations of 3-D geometry that can accurately describe any shape from a simple 2-D line, circle, arc, or curve to the most complex 3-D organic free-form surface or solid.

See this overview of NURBS geometry for more information.