Hĕm'i-hē'dral, a.

[Gr. ἡμι, half, and ἕδρα, seat, base, from ἕζεσθαι, to sit.] (Crystallog.) Having half of the similar parts in a crystal, 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 octohedron, it being contained under four of the planes of an octohedron. Dana.