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A closed curved surface that is donut-shaped with a hole in the middle. It can be formed by rotating a circle about an axis that lies in the same plane as the circle but does not cut it. A cross-section of the torus in a plane perpendicular to the axis is two concentric circles. A cross-section in any plane that contains the axis is a pair of congruent circles at equal distances on both sides of the axis.
The volume of the torus is 2
2r2d, and its surface area is 42rd, where r is the radius of the generating circle, and d is the distance of its center from the axis.
