Tet`ra*he"dron (?), n. [Tetra- + Gr. &?; seat, base, fr. &?; to sit.] (Geom.) A solid figure inclosed or bounded by four triangles.

&fist; In crystallography, the regular tetrahedron is regarded as the hemihedral form of the regular octahedron.

Tet`ra*he"dron (?), n. [Tetra- + Gr. &?; seat, base, fr. &?; to sit.] (Geom.) A solid figure inclosed or bounded by four triangles.

&fist; In crystallography, the regular tetrahedron is regarded as the hemihedral form of the regular octahedron.

• (geometry) a polyhedron with four faces; the regular tetrahedron, the faces of which are equal equilateral triangles, is one of the Platonic solids.