(last updated: 10/07/16)
Psi is an Offense stat that improves the damage you deal with Psi attacks; the higher your Psi score, the more damage your Psi attacks do. A higher Psi stat also tends to help you be more likely to Parry any Psi attacks that people send your way, assuming you have the Parry ability.
Psi attacks utilize Psycho Power energy. Unlike Chi, Psycho Power is a purely human force, rather than energy around the world that they learn how to manipulate. The effects are visually similar to Chi attacks, though often look a bit more spectacular, and the effects can vary somewhat.
Note that just as Chi users cannout harness Psycho Power, Psions cannot use Chi. A psion's Chi score will be 0 in all cases. As chi is foreign to a Psion, their AURA score will normally be 10 lower than the average of the rest of their statistics. It's covered in detail in our Psycho Power guide, but in extreme summary: having Psycho Power within you tends to make you unable to interact much with Chi.
For additional information, read Fighting Psi.