Frustum of a pyramid (or cone) is a portion of pyramid (or cone) included between the base and the section parallel to the base not passing through the vertex.

Frustum of a cone and frustum of a pyramid


Formula for Volume of a Frustum
The volume of a frustum is equal to one-third the product of the altitude and the sum of the upper base, the lower base, and the mean proportional between the bases. In symbols

$V = \frac{1}{3}\left( A_1 + A_2 + \sqrt{A_1A_2} \right)h$


See the derivation of formula for volume of a frustum.