Illustration of tetrahedron
tetrahedron
noun
tet·ra·he·dron ˌte-trə-ˈhē-drən 
plural tetrahedrons or tetrahedra ˌte-trə-ˈhē-drə
: a polyhedron that has four faces
Did you know?
The simplest tetrahedron is made of four equal-sided triangles: one is used as the base, and the other three are fitted to it and each other to make a pyramid. But the great pyramids of Egypt aren't tetrahedrons: they instead have a square base and four triangular faces, and thus are five-sided rather than four-sided.
Example Sentences
Recent Examples on the Web If the regular tetrahedron doesn’t tile space, the question becomes: Do any tetrahedra? 
Quanta Magazine, 9 Feb. 2021 Triangles and tetrahedra are examples of more general shapes called simplices, which can be defined in any dimension. Quanta Magazine, 13 Jan. 2015 That extraordinary hardness arises from a strong and inflexible structure: Five atoms form a tetrahedron and share electron pairs with each other.3. Gemma Tarlach, Discover Magazine, 23 Mar. 2018 With eight modules of tetrahedrons (triangular pyramids) made of translucent polyethelene and aluminum tubes that billow out from a symmetrical axis, the piece feels both organic and machinelike. Molly Glentzer, Houston Chronicle, 10 Jan. 2018 They are defined by symmetry, and the Greeks knew that there were only five possibilities meeting the criteria: the tetrahedron, cube, octahedron, dodecahedron and icosahedron. Eugenia Cheng, WSJ, 10 Aug. 2017 The new metropolis would consist of a giant tetrahedron—a pyramid made of equilateral triangles—a shape that Fuller, the popularizer of the geodesic dome, admired for its stability and symmetry. Rachel Riederer, New Republic, 29 May 2017 As Mars cooled internally and shrank, some faces of the martian tetrahedron slumped. David S. F. Portree, WIRED, 8 May 2012 See More
Quanta Magazine, 9 Feb. 2021 Triangles and tetrahedra are examples of more general shapes called simplices, which can be defined in any dimension. Quanta Magazine, 13 Jan. 2015 That extraordinary hardness arises from a strong and inflexible structure: Five atoms form a tetrahedron and share electron pairs with each other.3. Gemma Tarlach, Discover Magazine, 23 Mar. 2018 With eight modules of tetrahedrons (triangular pyramids) made of translucent polyethelene and aluminum tubes that billow out from a symmetrical axis, the piece feels both organic and machinelike. Molly Glentzer, Houston Chronicle, 10 Jan. 2018 They are defined by symmetry, and the Greeks knew that there were only five possibilities meeting the criteria: the tetrahedron, cube, octahedron, dodecahedron and icosahedron. Eugenia Cheng, WSJ, 10 Aug. 2017 The new metropolis would consist of a giant tetrahedron—a pyramid made of equilateral triangles—a shape that Fuller, the popularizer of the geodesic dome, admired for its stability and symmetry. Rachel Riederer, New Republic, 29 May 2017 As Mars cooled internally and shrank, some faces of the martian tetrahedron slumped. David S. F. Portree, WIRED, 8 May 2012 See More Word History
Etymology
New Latin, from Late Greek tetraedron, neuter of tetraedros having four faces, from Greek tetra- + hedra seat, face — more at sit
First Known Use
1570, in the meaning defined above