tetrahedron

noun

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

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