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A solid having four plane triangular faces; a triangular pyramid.

*‘In particular he identified the five elements, fire, earth, air, water and celestial matter with the five regular solids, the tetrahedron, cube, octahedron, icosahedron and the dodecahedron.’**‘Plato believed that atoms have the shapes of regular polyhedra: cubes, tetrahedrons, octahedrons, and so on.’**‘The new structure consists of 20 small crystalline tetrahedrons - pyramids with four triangular sides - that fit together a bit like sections of an orange.’**‘These five remarkable solids - tetrahedron, cube, octahedron, dodecahedron, icosahedron - have faces that are congruent equilateral polygons.’**‘He assigned the tetrahedron to fire, because the tetrahedron is the regular solid with the sharpest angles, and because fire is the most penetrating of elements.’**‘Whereas the tetrahedron is composed of four triangles, the pyramid, which looks a lot like it, is composed of four triangles and a square - the base.’**‘As water's temperature decreases to and falls below the normal freezing point, more and more of its molecules self-organize into clusters that have structures based on the tetrahedron, a pyramid with a triangular base.’**‘The robot is called ‘TETwalker’ for tetrahedral walker, because it resembles a tetrahedron.’**‘In the methane molecule CH 4, for instance, the carbon atom is located at the center of a tetrahedron with the four hydrogen atoms located at the four apexes of the tetrahedron.’**‘The four C-H bonds are directed towards the four corners of a tetrahedron, with carbon at its center and hydrogens at each vertex.’**‘When all four corners of the tetrahedra are shared, the mineral is called a tectosilicate.’**‘For example, the convex hull of a 3D molecule is a convex polytope rather than a polygon, and it is tessellated by tetrahedra rather than triangles.’**‘The silica tetrahedra may also form a sheet structure where three oxygen atoms of each tetrahedron are shared by adjacent tetrahedra.’**‘The tetrahedrite crystal on this specimen is a simple tetrahedron with very smooth, clean crystal faces and a bright, shiny metallic gray color (personal observation).’**‘With his geodesic dome, Fuller began to express a favorite geometric shape of his, the tetrahedron, and he put it everywhere he could.’**‘An artist can look at a mathematical shape and envision unlimited possibilities, even from a shape as seemingly simple as a tetrahedron, a trefoil knot, a Möbius strip, or a fractal surface.’**‘The strongest and most stable molecular structure is the three-dimensional tetrahedron, the configuration achieved with silicates.’**‘He examined a vectorial system of geometry, Energetic - Synergetic geometry, based on the tetrahedron which provides maximum strength with minimum structure.’**‘The Enterprise Day at Joseph Rowntree School also involved representatives from The Institute Of Civil Engineers who built a giant tetrahedron with their young charges.’**‘Quartz crystals are silicon atoms surrounded by a tetrahedron of oxygen atoms linked at shared corners.’*

**Origin**

Late 16th century from late Greek tetraedron, neuter (used as a noun) of tetraedros ‘four-sided’.

**Pronunciation**

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