noun, plural tetrahedrons, tetrahedra [te-truh-hee-druh] .
Geometry. a solid contained by four plane faces; a triangular pyramid.
any of various objects resembling a tetrahedron in the distribution of its faces or apexes.

1560–70; tetra- + -hedron, modeled on Late Greek tetráedron, noun use of neuter of tetráedros four-sided

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World English Dictionary
tetrahedron (ˌtɛtrəˈhiːdrən)
n , pl -drons, -dra
1.  See also polyhedron a solid figure having four plane faces. A regular tetrahedron has faces that are equilateral triangles
2.  any object shaped like a tetrahedron
[C16: from New Latin, from Late Greek tetraedron; see tetra-, -hedron]

Collins English Dictionary - Complete & Unabridged 10th Edition
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Word Origin & History

solid figure contained by four triangular surfaces, 1570, from Late Gk. tetraedron, originally neuter of tetraedros (adj.) "four-sided," from tetra- "four" + hedra "seat, base, chair, face of a geometric solid," from PIE base *sed- "to sit" (see sedentary).
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Science Dictionary
tetrahedron   (tět'rə-hē'drən)  Pronunciation Key 
Plural tetrahedrons or tetrahedra
A polyhedron having four faces.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences for tetrahedra
Structure the truncated tetrahedra are joined to each other via their hexagonal faces.
The octahedra are joined to the truncated tetrahedra via their triangular faces.
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