Unseen
From Tolkien Gateway
All things in Tolkien's world fall into the realm of either the Seen or the Unseen.
The Unseen are objects and creatures of the spirit world. Some of the most obvious examples of Unseen creatures are the Ringwraiths, which were invisible and formless without disguises provided by Sauron.
Wizards and some Elves have the ability to see and affect Unseen creatures.