dodecahedron - A polyhedron with twelve pentagonal faces. The regular dodecahedron is one of the five Platonic solids (along with the tetrahedron, hexahedron (cube), octahedron, and icosahedron. The faces of a regular dodecahedron are all regular pentagons. (pr. dohdek-e-hee"dren) The plural form can be either dodecahedrons or dodecahedra.