Psi is a term from parapsychology derived from the Greek, ψ psi, 23rd letter of the Greek alphabet; from the Greek ψυχή psyche, "mind, soul". The previous sentence is taken from the Wikipedia article on Psi(parapsychology) which appears perfectly adequate to describe this term. WikiPedia
Continuing with information from that article, we note: 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.)
Tom Campbell within his trilogy My Big TOE rather takes the position that such occurrences and abilities are not 'para' normal by any means but rather are perfectly normal and explainable in a Virtual Reality.