a. (sb.) Geom., Cryst., Nat. Hist., etc. Also triedral. [f. Gr. τρι-, TRI- + ἔδρα base + -AL.] Of a solid figure or body: Having three sides or faces (in addition to the base or ends); bounded laterally by three surfaces; triangular in section. Trihedral angle or quoin, a solid angle formed by three surfaces meeting at a point.
1789. A. Crawford, in Med. Commun., II. 355. Obtuse trihedral pyramids.
1812. Sir H. Davy, Chem. Philos., 125. Thus 6 particles may compose an octoedron or triedral prism.
1839–47. Todd’s Cycl. Anat., III. 267/2. The inferior molars are … divided into two triedral portions.
1878. Gurney, Crystallogr., 85. The trihedral quoins of the rhombic dodecahedron.
1880. Huxley, Crayfish, iii. 116. Each of these joints is trihedral, the outer face being convex; the inner, flat; and the upper concave.
B. sb. Geom. A trihedral figure; the figure determined by three planes meeting at a point (Cent. Dict. Suppl., 1909). Also Trihedron.
1828. Webster, Trihedron, a figure having three equal sides.
1860. Worcester cites Davies.
