In mathematics, a toroid is a doughnut-shaped object, such as an O-ring. It is a ring form of a solenoid. Its annular shape is generated by revolving a plane geometrical figure about an axis external to that figure which is parallel to the plane of the figure and does not intersect the figure. When a rectangle is rotated around an axis parallel to one of its edges, then a hollow cylinder (resembling a piece of straight pipe) is produced.
If the revolved figure is a circle, then the surface of such an object is known as a torus.
See also 
- The dictionary definition of toroid at Wiktionary
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