Sphere

Earth was an imperfect sphere. (TV: The Blood Line)

Nestene energy units were hollow spheres. (TV: Spearhead from Space)

Torchwood London referred to the Void Ship as "the sphere". (TV: Army of Ghosts)

The Toclafane appeared as human heads encased in metal spheres. (TV: The Sound of Drums/Last of the Time Lords)

Behind the scenes
A sphere is the set of all points equidistant from one particular point in three dimensional space.