What is "perispirit"

In Spiritism, "Perispirit" is the subtle body that is used by the spirit to connect with the perceptions created by the brain. The term is found among the extensive terminology originally devised by Allan Kardec in his books about Spiritism. Its first use was in a commentary (by Kardec) to the answer given by the spirits to the 93rd question of the Spirits Book:

Is the spirit, properly so called, without a covering, or is it, as some declare, surrounded by a substance of some kind? "The spirit is enveloped in a substance which would appear to you as mere vapor, but which, nevertheless, appears very gross to us, thought it is sufficiently vaporous to allow the spirit to float in the atmosphere, and to transport himself through space at pleasure." As the germ of a fruit is surrounded by the perisperm so the spirit, properly so called, is surrounded by an envelope which, by analogy, may be designated as the perispirit.

This was a breakthrough concept, as the spirit had not been thought of as usually from its form (the ghost one saw was considered "the ghost itself", not the appearance of a ghost). Kardec, then, was compelled to develop further the notion, especially by given "scientific" fundamentation to his theory. He studied the properties of what was then called "fluids" (electricity, magnetism, heat) and broadened the research towards those he termed "psychic" or "spiritual fluids". Both terms, especially the previous have stuck and are still used (or abused) up to now.