-hedron Also -hedra and -hedral.
A geometrical solid with a specified number of plane faces.
[Greek hedra, seat, base.]
A tetrahedron is a solid having four plane triangular faces, a triangular pyramid; similarly, other solids with increasing numbers of plane faces have names that are usually based on Greek number prefixes (see the entry Number words): octahedron (eight), decahedron (ten), dodecahedron (twelve), icosahedron (twenty). In general, a polyhedron is any solid figure with many plane faces. A few terms relate to the shape of the faces rather than their number: rhombohedron, one with six equal rhombuses as faces; trapezohedron, one those faces are trapeziums or trapezoids. Plurals are traditionally formed in -hedra (polyhedra, tetrahedra), but now often in -s instead. Related adjectives are formed in -hedral (octahedral, dodecahedral). For flat geometric figures, see -gon.