In the Western World, early philosophical theories of the nature of spirit conceived it as the vital principle – or breath of life – which both animated the body and mediated between the animal functions of the body and the Soul. By the Middle Ages this dual function had been much complexified, but from about the turn of the 17th century spirit, conceived as literal vital principle, had become a more general metaphor, sometimes interchangeable with the soul, sometimes – as in the Holy Spirit – taken to designate a "person" of the Trinity. By the 19th century it had become common to use the term "spirits" to describe disembodied souls (see Astral Body), quite possibly occupying a "sphere" of their own.
The term is often encountered in Horror and Supernatural Fiction, though less frequently in fantasy. It generally takes one of two broad senses: it describes disembodied vital principles which or who occupy regions unavailable to the senses, and which or who may or may not manifest the animate nature of the Universe; or it describes unhoused souls, who are also known as Ghosts. These two broad senses may be linked: humans, for instance, may discover upon dying (see Afterlife; Posthumous Fantasy) that they have entered (or returned to) a spirit world, which circumambiates and transcends our physical realm (see Spiritualism). [JC]