A sphere "globe, ball" is a perfectly round geometrical and circular object in three-dimensional space that resembles the shape of a completely round ball . Like a circle , which, in geometric contexts, is in two dimensions, a sphere is defined mathematically as the set of points that are all the same distance r from a given point in three-dimensional space. This distance r is the radius of the sphere, and the given point is the center of the sphere. The maximum straight distance through the sphere passes through the center and is thus twice the radius; it is the diameter .