In esoteric philosophies a plane, other than the physical plane is conceived as a subtle state of consciousness that transcends the known physical universe. The original source of the word "plane" in this context seems to be the late Neoplatonist Proclus, who refers to to platos, "breadth", as in the phrase en to psuchiko platei.
H. P. Blavatsky defined the word plane as follows:
Plane. From the Latin planus (level, flat) an extension of space or of something in it, whether physical or metaphysical, e.g., a “plane of consciousness”. As used in Occultism, the term denotes the range or extent of some state of consciousness, or of the perceptive power of a particular set of senses, or the action of a particular force, or the state of matter corresponding to any of the above.
- Dodds, cited in Poortman, 1978, vol II, 54
- Helena Petrovna Blavatsky, Theosophical Glossary (Krotona, CA: Theosophical Publishing House, 1973), 6.