The Collaborative International Dictionary of English v.0.48:

Tetrahedron \Tet`ra*he"dron\, n. [Tetra- + Gr. ? seat, base, fr.
   ? to sit.] (Geom.)
   A solid figure inclosed or bounded by four triangles.
   [1913 Webster]

   Note: In crystallography, the regular tetrahedron is regarded
         as the hemihedral form of the regular octahedron.
         [1913 Webster]

   Regular tetrahedron (Geom.), a solid bounded by four equal
      equilateral triangles; one of the five regular solids.
      [1913 Webster]

WordNet (r) 3.0 (2006):

tetrahedron
    n 1: any polyhedron having four plane faces