tetrahedron

definition: 
A solid having four plane triangular faces; a triangular pyramid
sentence: 
The new structure consists of 20 small crystalline tetrahedrons - pyramids with four triangular sides - that fit together a bit like sections of an orange. In designing aircraft his use of tetrahedral structural elements inspired him to develop and patent a system of space frame architecture.
difficulty: 
medium
toefl frequency: 
1
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