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This category contains definitions related to Spheres in the context of Geometry.
Related results can be found in Category:Spheres.

A sphere is a surface in solid geometry such that all straight lines falling upon it from one particular point inside it are equal.

In the words of Euclid:

When, the diameter of a semicircle remaining fixed, the semicircle is carried round and restored again to the same position from which it began to be moved, the figure so comprehended is a sphere.

(The Elements: Book $\text{XI}$: Definition $14$)