Up to recently it was customary to designate such correspondences as telepathic, clairvoyant, precognitive, psychokinetic, or by roughly equivalent terms. In the past few years though the term PSI has come into increasingly widespread use in relation to class of occult correspondences formerly encompassed by such terms as those given above.

Many investigators have come to feel that there are no compelling grounds for making any assumption other than that behind the various phenomenological forms and manifestations in the realm of occult correspondences there is one underlying process. The fundamental what is it, which they presume to be common noumenon behind the phenomena of telepathic, clairvoyant, precognitive, psychokinesis, etc., they agree to call PSI.

The term PSI succeeds in eliminating some of the possibly misleading implications like a PSI event, a PSI relationship, PSI phenomena, it is supposed to be understood that PSI refers to nothing more than, the still unknown means or process by which a given event, relationship, or phenomenon is brought about. It is often more or less automatically assumed, however, that this means or process is an actual something or other which in principle can be positively identified {that is, not solely by exclusion} and differentiated from a set of non-PSI processes.

The term PSI, in other words, is not limited to the designation merely of a class of events which leads itself to differentiation solely on certain phenomenological grounds from other classes of events. It is frequently used to refer to a quite special and even, as far as all the natural forces and processes now known to science go. A uniquely operating principle which is postulated to be the particular thing in itself that, working from below, as it were, is inherently responsible for the fact that this class of events can be more or less sharply differentiated from other classes of events.

Despite this lack of clarity surrounding the concept of PSI and the looseness with which the term is often used, in the minds of some Parapsychologists and Parapsychic Scientists it has come to be looked upon as a force of some kind-a spiritual force, a psychic force, a force vaguely associated with thought-at all events, a non-material force insofar as it is generally held to be independent of time and space. Some how, the notion of a PSI force has taken deep root and it is this force that in one form or another is held to provide the causal connection between the events that make up those correspondences that would otherwise be completely inexplicable.

Source for reference and recommended reading on this subject is as follows. Parapsychology and the Unconscious by, Jule Eisenbud, M.D.