tetrahedron Geom.
(tɛtrəˈhiːdrən, -ˈhɛdrən)
Pl. -a or -ons. Also 6–9 tetraedron; 6–8 tetra(h)edrum.
[ad. late Gr. τετράεδρον n., 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 {ltflat}s. 1800 tr. 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. |