hemihedral

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Exhibiting only half the faces required for complete symmetry. Used of a crystal.

from The Century Dictionary.

• In mineralogy, exhibiting hemihedrism; having, as a crystal, only half the number of planes belonging to any particular form which the law of symmetry requires.
• In mathematics, substituting negative for positive signs in regular alternation.
• Also hemihedric, hemiedric.
• In the relation of so-called hemihedral crystals to crystal symmetry, see symmetry.

from the GNU version of the Collaborative International Dictionary of English.

• adjective (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective of a crystal Having only half the plane faces needed for the highest degree of symmetry in its system