It seems to be assumed that I've heard of "psi". No definition is given, nor can I even find one on wikipedia. A search yields information about the unit of p

It seems to be assumed that I've heard of "psi". No definition is given, nor can I even find one on wikipedia. A search yields information about the unit of physical pressure. One of the examples given of a psi-related phenomenon is extra-sensory perception. As far as I know, ESP has never been rigorously demonstrated to exist, although it's been attempted many times. What's this about?