Psi is a term from parapsychology derived from the Greek, ψ psi, twenty-third letter of the Greek alphabet; from the Greek ψυχή psyche, “mind, soul”.  Traditionally the term has had two sub-categories:
- Psi-Gamma – Pertaining to paranormal cognition (ESP, remote viewing, etc.)
- Psi-Kappa – Pertaining to paranormal action (psychokinesis, etc.)
The term was coined by biologist Bertold P. Wiesner, and first used by psychologist Robert Thouless in a 1942 article published in the British Journal of Psychology. Psi was argued by Thouless and Wiesner to offer a non-theoretical manner of referring to extrasensory perception and psychokinesis, these terms being unjustifiably loaded with suggestions as to how the phenomena were caused or experienced.
Although Thouless and Wiesner were careful to offer psi as merely referring to certain paranormal activity worthy of study, it has come to connote the processes that somehow cause them, or a certain faculty of human psychology. In a 1994 paper in the Psychological Bulletin, Daryl J. Bem and Charles Honorton defined psi thus: The term psi denotes anomalous processes of information or energy transfer, processes such as telepathy or other forms of extrasensory perception that are currently unexplained in terms of known physical or biological mechanisms. The term is purely descriptive: It neither implies that such anomalous phenomena are paranormal nor connotes anything about their underlying mechanisms.
Similarly, according to the Parapsychological Association, psi can be used either as a noun or adjective to identify paranormal processes and paranormal causation; the two main categories of psi are psi-gamma (paranormal cognition; extrasensory perception and psi-kappa (paranormal action; psychokinesis), although the purpose of the term “psi” is to suggest that they might simply be different aspects of a single process, rather than distinct and essentially different processes.
However, it has been pointed out by parapsychologist Carl Williams: Historically, the study of psi has been viewed as existing far outside the normal boundaries and concerns of 19th – and 20th – century science. Whether proposing the possibility of contact with the dead or novel forms of communication, books with titles such as Beyond the Reach of Sense (Heywood, 1974) and Beyond Reality (Rogo, 1990) establish psi as an object of scrutiny out of this world; and to many the study of psi and its existence are seen as existing in the Margins of Reality (Jahn & Dunne, 1987).
In popular culture, “psi” have become synonymous with psychic and “psionic” abilities.
- What do parapsychologists study?, Parapsychological Association (2007-02-03)
- Psi – Glossary of Key Words Frequently Used in Parapsychology, Parapsychological Association (2007-01-29)
- Thouless, R. H. (1942). “Experiments on paranormal guessing”. British Journal of Psychology, 33, 15-27.
- Bem, D. J., & Honorton, C. (1994). Does psi exist? Replicable evidence for an anomalous process of information transfer. Psychological Bulletin’, 115, 4-18.