Mystical energy, here by it's Chinese name Chi. The Japanese Ki and the Arabic Bakara mean the same thing. In Swedish, it is called Kraften and alternate English names are Force or Mana.
Chi is a primary attribute and the governing attribute for Fortune, Fu, and Magic.
Chi attributes govern your ability to "tune in" and control the mystical forces of the universe. In a materialistic world (such as our own), there are few indications of Chi, and it is not very important. But many T3 campaigns take place in worlds where mystic powers are common and obvious.