A designation for both extrasensory perception (ESP) and psychokinesis (PK) as proposed in 1946 by the British psychologists Drs. Robert Thouless and W. P. Weisner. The reasons for their proposal were that "psi" is the twenty-third letter of the Greek alphabet commonly used in parapsychology to include both phenomena of ESP and PK because both are closely related. However, since that time the term often has been inaccurately used to include almost any paranormal experience or phenomenon.

Discarded theories:

Theories concerning the functioning of psi have been difficult to formulate because it defies most laboratory experiments to describe it activity in physical or quasi-physical terms. It operates outside of the boundaries of time and space. No physical variables influence psi in laboratory testing. Theories that psi is some sort of a wave, particle, force, or field have been advanced and discarded. Psi is not, nor is it affected by the four forces of physics; strong nuclear force, weak nuclear force, gravitational force, or electromagnetic force. It is not subject either to the law of thermodynamics or the law of gravity. Psi requires no exchange of energy, which is pretty remarkable in incidents of apparent PK; for example, according to the mechanical laws of physics, the dematerialization of a copper penny would require the energy of a small nuclear bomb. Psi defies the theory of relativity which states that no particle or object can move faster than the speed of light which is 186,000 miles per second.

All such psi defiance to be defined in physical terms has forced researches to look elsewhere for explanations. Some occultist believe psi is a vibration manifested throughout the world, but most scientists view this possibility with skepticism.