Dihedral

a. [Gr. δι, for δίς, twice, double, and ἕδρα, a seat, bottom, base, from ἕζεσθαι, to sit.] Having two plane faces; as, the dihedral summit of a crystal.

Dihedral angle, the angular space included between two planes which meet each other.

a. [Gr. δί, for dis, twice, double, and hexahedral, q. v.] (Crystallog.) Having the form of a hexahedral prism with trihedral summits.