An octahedron is a closed 3-d shape with 8 polygonal faces. There are 257 topologically different convex octahedra. The simplest octahedron to make is probably the heptagon based right pyramid. The net for this shape is a regular heptagon with seven identical triangles attached to its edges. If the triangles are folded to meet at a point above the heptagonal base you will have an octahedron.
An octahedron is a 3D shape with 8 sides.
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The suffix "hedron" implies 3D. The 2D suffix is "gon"
It is an octahedron that is a 3D shape with 8 faces