Tetrahedron \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.

Source: Webster's Revised Unabridged Dictionary (1913)