Two Pyramids symmetrically placed base-to-base, also called a Bipyramid. They are the Duals of the Archimedean Prisms.
See also Elongated Dipyramid, Pentagonal Dipyramid, Prism, Pyramid, Trapezohedron, Triangular Dipyramid, Trigonal Dipyramid
References
Cundy, H. and Rollett, A. Mathematical Models, 3rd ed. Stradbroke, England: Tarquin Pub., p. 117, 1989.