prismatoid

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A polyhedron all of whose vertices lie in one of two parallel planes.

from The Century Dictionary.

• noun 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.
• noun A solid having two parallel polygonal bases connected by triangular faces.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun mathematics Any polyhedron whose vertices all lie in either of two parallel planes

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun 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