Itten or One point in english, is a point on your body right below your belly button. This is an important thing in using ki, it is referred as the Don Ten in Chinese. It is the center of your body both physically and ki wise. This is the point where your ki originates. I think of it as a battery like your car battery, it is always charging as long as your body is "running". You can also watch your opponents itten, he cannot hit you very effectively without first moving his itten.