The name is taken from the Greek upper case delta (Δ), which has the shape of an equilateral triangle. In geometry, a deltahedron ( plural deltahedra) is a polyhedron whose faces are all equilateral triangles. This figure is not a strictly-convex deltahedron since coplanar faces are not allowed within the definition. The largest strictly-convex deltahedron is the regular icosahedron This is a truncated tetrahedron with hexagons subdivided into triangles.