Frustrum

A frustum is the 3D shape that remains when a cone is cut parallel to its base and the top cone removed.

The word frustum is a Latin word that means piece or bit. 

The frustum is the shape that remains once the smaller cone has been cut off

The volume of the frustum is easy to calculate. It is just the volume of the original cone minus the volume of the smaller cone.

The original cone and the smaller cone that is removed are always similar.