From Tales of Maj'Eyal
Psi is the resource used by Psionic classes (Solipsist, Mindslayer, and Possessor), as well as Psyshot. It slowly regenerates over time, can be restored by talents and some equipment. Players can increase psi pool by raising willpower (+1 per point invested in willpower except for the Solipsist class). Psi is doubly affected by fatigue.