The Collaborative International Dictionary of English v.0.48:

Deltohedron \Del`to*he"dron\, n. [Gr. de`lta, the letter [Delta]
   + 'e`dra seat, base.] (Crystallog.)
   A solid bounded by twelve quadrilateral faces. It is a
   hemihedral form of the isometric system, allied to the
   tetrahedron.
   [1913 Webster]