Wiktionary
n. A dipyramid.
Wikipedia
bgcolor=#e7dcc3 colspan=2|Set of regular right bipyramids
align=center colspan=2|
(Example hexagonal form)
bgcolor=#e7dcc3|Coxeter diagram
bgcolor=#e7dcc3| Schläfli symbol
bgcolor=#e7dcc3|Faces
bgcolor=#e7dcc3|Edges
bgcolor=#e7dcc3|Vertices
bgcolor=#e7dcc3| Face configuration
bgcolor=#e7dcc3| Symmetry group
bgcolor=#e7dcc3| Rotation group
bgcolor=#e7dcc3| Dual polyhedron
bgcolor=#e7dcc3|Properties
bgcolor=#e7dcc3| Net
An n-gonal bipyramid or dipyramid is a polyhedron formed by joining an n-gonal pyramid and its mirror image base-to-base. An n-gonal bipyramid has 2n triangle faces, 3n edges, and 2 + n vertices.
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.