From Bulbapedia, the community-driven Pokémon encyclopedia.

The reverse world (Japanese: 反転世界 reverse world) is another universe directly opposite the Pokémon world. It is where Giratina resides and plays a role in Giratina and the Sky's Bouquet: Shaymin.

It is depicted as a world where there is no solid ground where it might be expected and mirror images of scenery is in vertical symmetry with itself.