1) Dipyramid 
An n-gonal bipyramid or dipyramid is a polyhedron formed by joining an n-gonal pyramid and its mirror image base-to-base. The referenced n-gon in the name of the bipyramids is not an external face but an internal one, existing on the primary symmetry plane which connects the two pyramid halves. The face-transitive bipyramids are the dual polyhedra...
Crystal shape in form with a plane dividing a crystal into two pyramids base to base. Crystals exhibiting this are described as bipyramidal.
a form symmetrical about a plane dividing it into two pyramids. Also called