Source : Webster's Revised Unabridged Dictionary (1913)

Dihedral \Di*he"dral\, a. [Gr. di- = di`s- twice + ? a seat,
   bottom, base, fr. ? to sit. Cf. {Diedral}.]
   Having two plane faces; as, the dihedral summit of a crystal.

   {Dihedral angle}, the angular space contained between planes
      which intersect. It is measured by the angle made by any
      two lines at right angles to the two planes.