(last updated: 12/28/17)
The Chi stat is an Offensive one that improves the damage you do with Chi attacks; higher Chi scores are better than low ones in this regard. A high Chi score also helps you be more likely to succeed in Parrying a Chi attack someone does to you, assuming you have the Parry ability.
Chi attacks are ones that utilize Chi energy, the 'latent energy of the world' that almost anyone can tap into with sufficient training. Chi attacks are noticable due to the bright energy glows that are caused by the use of Chi, and are the most common kind of Projectile, though other uses are certainly possible.
A character who is even minutely proficient with Chi is unable to tap into Psycho Power, and their PSI score will be a 0 to reflect this. Chi users are further unable to defend as easily against Psycho Power, and normally have WILL scores reflecting this; a chi user's WILL is typically 10 lower than the average of his other stats.
A guide has been created to further explain chi, but this is quite expansive and optional reading. Understand that attempts at unique or questionable displays of chi will be subject to the information in this extended guide.
For additional information, read Fighting Chi.