In geometry, a pentahedron (plural: pentahedra) is a polyhedron with five faces. Since there are no face-transitive polyhedra with five sides and there are two distinct topological types, this term is less frequently used than tetrahedron or octahedron.With regular polygon faces, the two topological forms are the square pyramid and triangular prism. Geometric variations with irregular faces can also be constructed.The square pyramid can be seen as a degenerate triangular prism where one edge of its side edges is collapsed into a point, losing one edge and one vertex, and changing two squares into triangles.An irregular pentahedron can be a non-convex solid.