Torus

A torus is a surface formed by revolving a circle in three-dimensional space through one full turn around an axis that lies in the same plane as the circle but does not pass through it. The most familiar form, the ring torus, has the shape of a doughnut or an inner tube, with a major radius measured from the center of the whole shape to the center of the tube and a minor radius measuring the tube itself. Topologically a ring torus is the product of two circles, making it a compact two-dimensional surface of genus one. When the two radii are equal the surface pinches to a point at its center, called a horn torus, and when the minor radius exceeds the major radius the surface self-intersects, called a spindle torus.