Geom. Pl. -a or -ons. Also 6–9 tetraedron; 6–8 tetra(h)edrum. [ad. late Gr. τετράεδρον sb., prop. neut. of τετράεδρος adj. four-sided, f. τετρα- four + ἕδρα base.] A solid figure contained by four plane triangular faces, a triangular pyramid; spec. the regular tetrahedron, the first of the five regular solids, contained by four equilateral triangles. Hence, any solid body, esp. a crystal, of this form.
Orthogonal tetrahedron, one in which the opposite edges, taken in pairs, are at right angles to one another. Polar tetrahedron, one of which the faces are polar to the vertices of another tetrahedron.
1570. Billingsley, Euclid, XI. def. xxii. 319. A Tetrahedron is a solide which is contained vnder fower triangles equall and equilater.
1571. Digges, Pantom., IV. T ij. Tetraedron … a body Geometricall. Ibid. margin. Tetraedrum.
1653. H. More, Antid. Ath., I. vii. § 5. The notion or idea of God … is no more arbitrarious or fictitious than the notion of a cube or tetraedrum or any other of the regular bodies in Geometry.
1706. W. Jones, Syn. Palmar. Matheseos, 234. The Tetraedrum of 4 solid ∠s.
1800. trans. Lagrange’s Chem., I. 359. Susceptible of crystallizing in tetraedra.
1875. Bennett & Dyer, Sachs’ Bot., 50. They [crystalloids] appear as cubes, tetrahedra, octohedra, rhombohedra, and in other forms.
1878. Gurney, Crystallogr., 92. Tetrahedrons are contained by four equiangular triangles.
