The Collaborative International Dictionary of English v.0.48:

Triedral \Tri*e"dral\, a.
   See Trihedral.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Trihedral \Tri*he"dral\, a. [See Trihedron.] (Geom.)
   Having three sides or faces; thus, a trihedral angle is a
   solid angle bounded by three plane angles. [Written also
   triedral.]
   [1913 Webster]