在几何学中，九面体是指由9个平面组成的多面体，而边长全部等长的九面体是七角柱是一种半正多面体。在九面体中，四角锥柱和它的对偶多面体都是九面体。九面体可以是七角柱、八角锥、双三角锥柱等多面体。

In geometry, a enneahedron (or nonahedron) is a polyhedron with nine faces. There are 2606 topologically distinct enneahedra and none are regular, so this name is ambiguous.