Source : Webster's Revised Unabridged Dictionary (1913)

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 : WordNet®

regular tetrahedron
     n : a tetrahedron with four equilateral triangular faces