n. [ Tetra- + Gr. &unr_; seat, base, fr. &unr_; to sit. ] (Geom.) A solid figure inclosed or bounded by four triangles. [ 1913 Webster ]
☞ In crystallography, the regular tetrahedron is regarded as the hemihedral form of the regular octahedron. [ 1913 Webster ]
Regular tetrahedron (Geom.),