Is a icosahedron a polyhedron or non-polyhedron?

A polyhedron is a closed figure with several (planar) sides, and all its sides are planar. A non polyhedron does not have these properties, maybe it is not closed, maybe one or more of the sides is a...

A polyhedron has "many sides" (by definiton). The only 3-D shape which does not have "many sides" is a sphere (which technically only have one "side").

A non polyhedron does not have these properties, maybe it is not closed, maybe one or more of the sides is a curved surface. In some branches of topology the sides of a polyhedron cannot intersect...

The sides of a rectangle are not all equal so a rectangle is not a regular polygon. And, as a consequence, any 3-dimensional shape made from rectangles cannot be a regular polyhedron.

A Polyhedron is a closed plane figure whose faces are portions of planes. Prisms and pyramids are examples of Polhedron's. While a Regular Polthedron is a Polyhedron whose facces are all regular...