Source : Webster's Revised Unabridged Dictionary (1913)

Hemihedral \Hem`i*he"dral\, a. [Hemi- + Gr. ? seat, base, fr. ?
   to sit.] (Crystallog.)
   Having half of the similar parts of a crystals, instead of
   all; consisting of half the planes which full symmetry would
   require, as when a cube has planes only on half of its eight
   solid angles, or one plane out of a pair on each of its
   edges; or as in the case of a tetrahedron, which is
   hemihedral to an octahedron, it being contained under four of
   the planes of an octahedron. -- {Hem`i*he"dral*ly}, adv.