The Shadow-realm, Spirit-Realm or Shadow-world (Q."Hóména") was a part of Arda or Eä which semed sealed for Hildor and maybe Dwarves, but open to the Immortals to whom it seemed like just a different view or aspect of the living World.Undead however, or mortals who had entered the Shadow world via magic (such as ring of power for example), experienced it as a world entirely different to the world they knew and expierenced as the real world. The spirits of mortal men or orcs or elves who were tied to the matter of Arda via sorcery or had rejected to go to the Halls of Mandos also seemingly populated a part of this realm which was known as the Wraith-world (Q."Fairëména"). However mortals, even if they would not realize it, did take part in the Shadow-world to some degree and would appear as a faint glow to those who entered it while those Unseen, who existed only as bodily shapes of shadows, spectres or invisible matter, could be seen in their true, often horrible and twisted shape.
The Spirit- or Twilight-world could in some ways be explained as an additional dimension or plane of existence.