Tetrahedron \Tet`ra*he"dron\, n. [Tetra- + Gr. ? seat, base, fr.
   ? to sit.] (Geom.)
   A solid figure inclosed or bounded by four triangles.
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   Note: In crystallography, the regular tetrahedron is regarded
         as the hemihedral form of the regular octahedron.
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   Regular tetrahedron (Geom.), a solid bounded by four equal
      equilateral triangles; one of the five regular solids.
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	tetrahedron
    n 1: any polyhedron having four plane faces