Definition:Sphere/Geometry

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

Radius

 * Sphere.png

Thus a sphere is the three-dimensional equivalent of the circle.

Every point on the sphere is at the same distance from its center.

Note
As the sphere is defined here, it is specified as being the surface only, that is, not the inside.