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.