a. Also 8–9 tetraedral. [f. late Gr. τετράεδρος (see TETRAHEDRON) + -AL.]
1. a. Having four sides (in addition to the base or ends); enclosed or contained laterally by four plane surfaces, as a tetrahedral prism or pyramid. Tetrahedral angle, quoin, one bounded by four planes meeting at a common apex.
1794. G. Adams, Nat. & Exp. Philos., II. xiv. 46. The internal cavity is found to be lined with beautiful tetrahedral prisms.
1812. Sir H. Davy, Chem. Philos., 124. Four particles may compose a tetrahedron, five a tetraedral pyramid, six an octaedron.
1828. Stark, Elem. Nat. Hist., II. 139. Body tetraedral, furrowed above.
1878. Gurney, Crystallogr., 85. The tetrahedral quoins … of the rhombic dodecahedron.
b. Quadrilateral, quadrangular. (Also in comb.)
1816. Kirby & Sp., Entomol., xxvii. (1818), II. 491. Cells with regular tetrahedral bottoms. Ibid., 494. The tetrahedral-bottomed transition cells … still preserved their usual shape of hexagonal prisms.
2. Of or pertaining to a tetrahedron; having the form of a tetrahedron; spec. in Cryst., belonging to a division of the isometric system of which the regular tetrahedron is the characteristic form.
1805–17. R. Jameson, Char. Min. (ed. 3), 200. Tetrahedral (Haūy tetraēdre), when the crystal has the regular tetrahedron as a secondary form. Example, Tetrahedral blende.
1876. Harley, Mat. Med. (ed. 6), 369. The spores are minute, tetrahedral granules, each presenting four facets, and are minutely ridged by a hexagonal network.
1903. A. Graham Bell, in Nat. Geog. Mag., June, 225. The Tetrahedral principle in Kite Structure. When a tetrahedral frame is provided with aero-surfaces of silk or other material … it becomes a tetrahedral kite, or kite having the form of a tetrahedron.
Hence Tetrahedrally adv., in a tetrahedral manner or form. So Tetrahedric, Tetrahedrical adjs., tetrahedral.
1860. Mayne, Expos. Lex., Tetrahedricus,… tetrahedrical.
1864. Webster, Tetrahedrally (citing Dana).
1882. Vines, Sachs’ Bot., 13. The four spores or pollen-grains do not lie in one plane but are arranged tetrahedrally, and have moreover a somewhat tetrahedral form. Ibid., 438.
1890. Smithsonian Rep., 367. This latter [double linking] is an immediate consequence of the tetrahedric conception.
