A sphere was a round object.
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)