**Toroid**

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In mathematics, a **toroid** is a surface of revolution with a hole in the middle, like a doughnut. The axis of revolution passes through the hole and so does not intersect the surface. For example, when a rectangle is rotated around an axis parallel to one of its edges, then a hollow rectangle-section ring is produced. If the revolved figure is a circle, then the object is called a torus.
The term "**toroid**" Is also used to describe a toroidal polyhedron. In this context a **toroid** need not be circular and may have any number of holes. Wikipedia, Toroid

See Also

**Torus**
**Vortex**
**The Nature and Dynamics of Vibration and Toroids**