What is "tetrahedral angle"

Tetrahedral \Tet`ra*he"dral\, a. [See Tetrahedron.]

1. Having, or composed of, four sides.

2. (Crystallog.)

   1. Having the form of the regular tetrahedron.

   2. Pertaining or related to a tetrahedron, or to the system of hemihedral forms to which the tetrahedron belongs.

      Tetrahedral angle (Geom.), a solid angle bounded or inclosed by four plane angles.