tetrahedron

• tetraedrum (obsolete)

From Ancient Greek τετράεδρον (tetráedron, “triangle-based pyramid”), from τετράεδρος (tetráedros), from τετράς (tetrás, “four”) + ἕδρα (hédra, “seat”). By surface analysis, tetra- +‎ -hedron.

• enPR: tĕtrəhē'drən, IPA^(key): /tɛtɹəˈhiːdɹən/

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tetrahedron (plural tetrahedrons or tetrahedra)

1. (geometry) A polyhedron with four faces; the regular tetrahedron, the faces of which are equal equilateral triangles, is one of the Platonic solids.