In Tolkien's cosmology, rational living creatures such as Elves and Men were composed of two vital elements: the hröa and the fëa. The hröa was the material body, and the fëa was the immaterial essence, a concept translated by the English word 'spirit'.

After death, a spirit would be called to the Halls of Mandos in Valinor (though that summons could be refused). Once there, the fates of the spirits of Elves and Men were very different. The fëar of Elves were bound to Arda until it came to its ultimate end, and even after death they could be re-embodied in the West. For Men, though, there was no return within the confines of the world, and their spirits set out from Arda on a journey to an unknown destiny.

The Valar and the Maiar had spirits, too, but of a different nature to the Children of Ilúvatar. Indeed, from a certain perspective these beings simply were spirits, though they were able to appear in an embodied form - called a fana - at any time they wished.