prismatoid

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A polyhedron all of whose vertices lie in one of two parallel planes.

Century Dictionary and Cyclopedia

1. n. A solid having two parallel polygonal bases connected by triangular faces. If A and C are the areas of the bases of a prismatoid, and B that of the section halfway between them, then, h being the altitude, August's formula for the solid contents is ⅙ h (A + 4B + C).
2. n. A term used by N. Story Maskelyne for a crystal form whose faces are parallel to one of the crystallographic axes while meeting the other two axes.

Wiktionary

1. n. mathematics Any polyhedron whose vertices all lie in either of two parallel planes

WordNet 3.0

1. n. a polyhedron whose vertices all lie in one or the other of two parallel planes; the faces that lie in those planes are the bases of the prismatoid