\Tet`ra*he"dron\, n. [Tetra- + Gr. ? seat, base, fr.
? to sit.] (Geom.)
A solid figure inclosed or bounded by four triangles.
Note: In crystallography, the regular tetrahedron is regarded
      as the hemihedral form of the regular octahedron.
{Regular tetrahedron} (Geom.), a solid bounded by four equal
   equilateral triangles; one of the five regular solids.