A frustum is part of a solid cut off between two parallel planes; in particular, for a cone or a pyramid, a frustum is determined by the plane of the base and a plane parallel to the base.

 

The frustum of a sphere or spheroid is any part of these solids comprised between two circular sections; and the middle frustum of a sphere is that whose ends are equal circles, having the center of the sphere in the middle of it, and equally distant from both ends.

 

Frustum is Latin for "a piece broken off."